irqchip/brcmstb-l2: Make two init functions static

Fix sparse warnings:

drivers/irqchip/irq-brcmstb-l2.c:278:12: warning:
 symbol 'brcmstb_l2_edge_intc_of_init' was not declared. Should it be static?
drivers/irqchip/irq-brcmstb-l2.c:285:12: warning:
 symbol 'brcmstb_l2_lvl_intc_of_init' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>