dt-bindings: mtk-sysirq: Add multiple bases support for Mediatek sysirq

This describes how to specify multiple base addresses for sysirq
in mediatek platforms.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mars Cheng <mars.cheng@mediatek.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>