fix(hp/linux): initialize the C906-internal PLIC through OpenSBI

The retrosoc_hp platform registered no irqchip_init hook, so the
C906-internal PLIC (thead,c900-plic, decoded by the core BIU rather than
the SoC fabric) was never initialized before the kernel probed it. Select
FDT_IRQCHIP_PLIC and register fdt_irqchip_init: OpenSBI v1.9's
thead,c900-plic match applies the T-Head M-to-S delegation quirk that
S-mode external-interrupt delivery requires. Verified by nm on the
rebuilt fw_jump.elf (fdt_irqchip_init/fdt_irqchip_plic linked in).

app/ports/linux/opensbi/retrosoc_hp/Kconfig

@@ -2,5 +2,8 @@
 
 config PLATFORM_RETROSOC_HP
 	bool
+	select FDT
+	select FDT_IRQCHIP
+	select FDT_IRQCHIP_PLIC
 	select IPI_MSWI
 	default y

app/ports/linux/opensbi/retrosoc_hp/platform.c

@@ -8,6 +8,7 @@
 #include <sbi/sbi_platform.h>
 #include <sbi/sbi_timer.h>
 #include <sbi_utils/ipi/aclint_mswi.h>
+#include <sbi_utils/irqchip/fdt_irqchip.h>
 
 /* Internal C906 CLINT (thead,c900-clint), decoded by the core BIU. */
 #define RETROSOC_HP_CLINT_BASE           0x0C000000UL
@@ -140,6 +141,10 @@ static int retrosoc_hp_timer_init(void) {
 
 static const struct sbi_platform_operations s_platform_operations = {
     .early_init = retrosoc_hp_early_init,
+    /* Initialize the C906-internal PLIC from the boot FDT: the thead,c900-plic
+     * match applies the T-Head M-to-S delegation quirk that S-mode interrupt
+     * delivery requires. */
+    .irqchip_init = fdt_irqchip_init,
     .timer_init = retrosoc_hp_timer_init,
 };