irqchip: mtk-sysirq: dt-bindings: Add bindings for mediatek sysirq

Add binding documentation for Mediatek SoC SYSIRQ.

Signed-off-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
Link: https://lkml.kernel.org/r/1416902662-19281-5-git-send-email-yingjoe.chen@mediatek.com


Signed-off-by: Jason Cooper <jason@lakedaemon.net>