lib: utils/irqchip: Use fdt_driver for initialization

The irqchip driver subsystem does not need any extra data, so it can use
`struct fdt_driver` directly. The generic fdt_irqchip_init() performs a
best-effort initialization of all matching DT nodes.

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

lib/utils/irqchip/fdt_irqchip_imsic.c

@@ -92,7 +92,7 @@ static const struct fdt_match irqchip_imsic_match[] = {
 	{ },
 };
 
-struct fdt_irqchip fdt_irqchip_imsic = {
+const struct fdt_driver fdt_irqchip_imsic = {
 	.match_table = irqchip_imsic_match,
-	.cold_init = irqchip_imsic_cold_init,
+	.init = irqchip_imsic_cold_init,
 };