irqchip/dw-apb-ictl: Refactor priot to introducing hierarchical irq domains

Add the required abstractions that will help introducing hierarchical
domain support to the dw-apb-ictl driver.

No functional change.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
[maz: commit message, some cleanups]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Tested-by: Haoyu Lv <lvhaoyu@huawei.com>
Link: https://lore.kernel.org/r/20200924071754.4509-3-thunder.leizhen@huawei.com