platform: Drop irqchip warm init and exit hooks

Now that driver lifecycle is managed from within the SBI irqchip core,
platforms need only to initialize the driver once during cold init.
Remove the remaining platform hooks that are no longer used.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

lib/utils/irqchip/fdt_irqchip_plic.c

@@ -108,6 +108,4 @@ static const struct fdt_match irqchip_plic_match[] = {
 struct fdt_irqchip fdt_irqchip_plic = {
 	.match_table = irqchip_plic_match,
 	.cold_init = irqchip_plic_cold_init,
-	.warm_init = NULL,
-	.exit = NULL,
 };