dt-bindings: interrupt-controller: Add MStar interrupt controller

Add binding for MStar interrupt controller.

Signed-off-by: Mark-PK Tsai <mark-pk.tsai@mediatek.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200902063344.1852-3-mark-pk.tsai@mediatek.com