irqchip/mips-gic: Make local symbols static

The sparse tool complains as follows:

drivers/irqchip/irq-mips-gic.c:49:1: warning:
 symbol '__pcpu_scope_pcpu_masks' was not declared. Should it be static?
drivers/irqchip/irq-mips-gic.c:620:6: warning:
 symbol 'gic_ipi_domain_free' was not declared. Should it be static?
drivers/irqchip/irq-mips-gic.c:634:5: warning:
 symbol 'gic_ipi_domain_match' was not declared. Should it be static?

Those symbols are not used outside of irq-mips-gic.c, so marks
them static.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200714142245.16124-1-weiyongjun1@huawei.com