genirq: Export irq_gc_{unmask_enable,mask_disable}_reg

In order to allow drivers/irqchip/irq-brcmstb-l2.c to be built as a
module we need to export: irq_gc_unmask_enable_reg() and
irq_gc_mask_disable_reg().

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211020184859.2705451-8-f.fainelli@gmail.com