genirq/generic_chip: Export irq_unmap_generic_chip

Some irq controllers have to re-implement a private version for
irq_generic_chip_ops, because they have a different xlate to translate
hwirq. Export irq_unmap_generic_chip to allow reusing in drivers.

Signed-off-by: Jianmin Lv <lvjianmin@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/1658314292-35346-5-git-send-email-lvjianmin@loongson.cn