nios2: constify irq_domain_ops

struct irq_domain_ops is not modified, so it can be made const.

Signed-off-by: Tobias Klauser <[e-mail]>
Acked-by: Ley Foon Tan <[e-mail]>

arch/nios2/kernel/irq.c

···
 	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,
 };