irqchip/irq-mst: Add MStar interrupt controller support

Add MStar interrupt controller support using hierarchy irq
domain.

Signed-off-by: Mark-PK Tsai <mark-pk.tsai@mediatek.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Tested-by: Daniel Palmer <daniel@thingy.jp>
Link: https://lore.kernel.org/r/20200902063344.1852-2-mark-pk.tsai@mediatek.com