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>