nios2: constify irq_domain_ops

struct irq_domain_ops is not modified, so it can be made const.
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Ley Foon Tan <ley.foon.tan@intel.com>

arch/nios2/kernel/irq.c

@@ -67,7 +67,7 @@ static int irq_map(struct irq_domain *h, unsigned int virq,
 	return 0;
 }
 
-static struct irq_domain_ops irq_ops = {
+static const struct irq_domain_ops irq_ops = {
 	.map	= irq_map,
 	.xlate	= irq_domain_xlate_onecell,
 };