docs: add licensing information and attribution comments to various RTL and test files

- Updated multiple RTL files in the  and  directories to include licensing information under the Mulan PSL v2.
- Added attribution comments for contributions from ETH Zurich and University of Bologna in .
- Enhanced test files in the  directory with licensing details and ensured compliance with the Solderpad Hardware License, Version 0.51.
- Introduced a new test in  to verify the preservation of attribution for PULP HyperBus sources.

tests/rtl/ics55_sram_4096x32_stub.sv

@@ -1,5 +1,12 @@
 // Copyright (c) 2026 Yuchi Miao <miaoyuchi@ict.ac.cn>
 // retroSoC is licensed under Mulan PSL v2.
+// You can use this software according to the terms and conditions of the Mulan PSL v2.
+// You may obtain a copy of Mulan PSL v2 at:
+//             http://license.coscl.org.cn/MulanPSL2
+// THIS SOFTWARE IS PROVIDED ON AN "AS IS" BASIS, WITHOUT WARRANTIES OF ANY KIND,
+// EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+// See the Mulan PSL v2 for more details.
 
 module SRAM_4096X32_M8_BW (
     input  logic [11:0] A,

tests/rtl/opipsram_tb.sv

@@ -1,5 +1,12 @@
 // Copyright (c) 2026 Yuchi Miao <miaoyuchi@ict.ac.cn>
 // retroSoC is licensed under Mulan PSL v2.
+// You can use this software according to the terms and conditions of the Mulan PSL v2.
+// You may obtain a copy of Mulan PSL v2 at:
+//             http://license.coscl.org.cn/MulanPSL2
+// THIS SOFTWARE IS PROVIDED ON AN "AS IS" BASIS, WITHOUT WARRANTIES OF ANY KIND,
+// EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+// See the Mulan PSL v2 for more details.
 
 `timescale 1ns / 1ps
 

tests/rtl/psram_tb.sv

@@ -1,5 +1,12 @@
 // Copyright (c) 2026 Yuchi Miao <miaoyuchi@ict.ac.cn>
 // retroSoC is licensed under Mulan PSL v2.
+// You can use this software according to the terms and conditions of the Mulan PSL v2.
+// You may obtain a copy of Mulan PSL v2 at:
+//             http://license.coscl.org.cn/MulanPSL2
+// THIS SOFTWARE IS PROVIDED ON AN "AS IS" BASIS, WITHOUT WARRANTIES OF ANY KIND,
+// EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+// See the Mulan PSL v2 for more details.
 
 `timescale 1ns / 1ps
 

tests/rtl/usb2_descriptor_tb.sv

@@ -1,5 +1,12 @@
 // Copyright (c) 2026 Yuchi Miao <miaoyuchi@ict.ac.cn>
 // retroSoC is licensed under Mulan PSL v2.
+// You can use this software according to the terms and conditions of the Mulan PSL v2.
+// You may obtain a copy of Mulan PSL v2 at:
+//             http://license.coscl.org.cn/MulanPSL2
+// THIS SOFTWARE IS PROVIDED ON AN "AS IS" BASIS, WITHOUT WARRANTIES OF ANY KIND,
+// EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+// See the Mulan PSL v2 for more details.
 
 module usb2_descriptor_tb;
   logic                       [31:0] s_buffer_addr;

tests/rtl/usb2_packet_ram_tb.sv

@@ -1,5 +1,12 @@
 // Copyright (c) 2026 Yuchi Miao <miaoyuchi@ict.ac.cn>
 // retroSoC is licensed under Mulan PSL v2.
+// You can use this software according to the terms and conditions of the Mulan PSL v2.
+// You may obtain a copy of Mulan PSL v2 at:
+//             http://license.coscl.org.cn/MulanPSL2
+// THIS SOFTWARE IS PROVIDED ON AN "AS IS" BASIS, WITHOUT WARRANTIES OF ANY KIND,
+// EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+// See the Mulan PSL v2 for more details.
 
 module usb2_packet_ram_tb;
   logic        s_clk;

tests/rtl/usb2_pkg_tb.sv

@@ -1,5 +1,12 @@
 // Copyright (c) 2026 Yuchi Miao <miaoyuchi@ict.ac.cn>
 // retroSoC is licensed under Mulan PSL v2.
+// You can use this software according to the terms and conditions of the Mulan PSL v2.
+// You may obtain a copy of Mulan PSL v2 at:
+//             http://license.coscl.org.cn/MulanPSL2
+// THIS SOFTWARE IS PROVIDED ON AN "AS IS" BASIS, WITHOUT WARRANTIES OF ANY KIND,
+// EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+// See the Mulan PSL v2 for more details.
 
 module usb2_pkg_tb;
   logic [15:0] s_crc16;

tests/rtl/usb2_reg_tb.sv

@@ -1,5 +1,12 @@
 // Copyright (c) 2026 Yuchi Miao <miaoyuchi@ict.ac.cn>
 // retroSoC is licensed under Mulan PSL v2.
+// You can use this software according to the terms and conditions of the Mulan PSL v2.
+// You may obtain a copy of Mulan PSL v2 at:
+//             http://license.coscl.org.cn/MulanPSL2
+// THIS SOFTWARE IS PROVIDED ON AN "AS IS" BASIS, WITHOUT WARRANTIES OF ANY KIND,
+// EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+// See the Mulan PSL v2 for more details.
 
 `include "usb2_define.svh"
 

tests/rtl/usb2_sie_tb.sv

@@ -1,5 +1,12 @@
 // Copyright (c) 2026 Yuchi Miao <miaoyuchi@ict.ac.cn>
 // retroSoC is licensed under Mulan PSL v2.
+// You can use this software according to the terms and conditions of the Mulan PSL v2.
+// You may obtain a copy of Mulan PSL v2 at:
+//             http://license.coscl.org.cn/MulanPSL2
+// THIS SOFTWARE IS PROVIDED ON AN "AS IS" BASIS, WITHOUT WARRANTIES OF ANY KIND,
+// EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+// See the Mulan PSL v2 for more details.
 
 module usb2_sie_tb;
   logic        s_clk;