irqchip/irq-bcm7038-l1: Make bcm7038_l1_of_init() static

Fix the following sparse warning:

drivers/irqchip/irq-bcm7038-l1.c:419:12: warning: symbol
'bcm7038_l1_of_init' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200417074036.46594-1-yanaijie@huawei.com