irqchip/mmp: Make mmp_irq_domain_ops static

Fix sparse warning:

drivers/irqchip/irq-mmp.c:182:29: warning:
 symbol 'mmp_irq_domain_ops' was not declared. Should it be static?
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

drivers/irqchip/irq-mmp.c

@@ -176,7 +176,7 @@ static int mmp_irq_domain_xlate(struct irq_domain *d, struct device_node *node,
 	return 0;
 }
 
-const struct irq_domain_ops mmp_irq_domain_ops = {
+static const struct irq_domain_ops mmp_irq_domain_ops = {
 	.map		= mmp_irq_domain_map,
 	.xlate		= mmp_irq_domain_xlate,
 };