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

Documentation/devicetree/bindings/arm/mediatek/mediatek,sysirq.txt

+Mediatek 65xx/81xx sysirq
+
+Mediatek SOCs sysirq support controllable irq inverter for each GIC SPI
+interrupt.
+
+Required properties:
+- compatible: should be one of:
+	"mediatek,mt8135-sysirq"
+	"mediatek,mt8127-sysirq"
+	"mediatek,mt6589-sysirq"
+	"mediatek,mt6582-sysirq"
+	"mediatek,mt6577-sysirq"
+- interrupt-controller : Identifies the node as an interrupt controller
+- #interrupt-cells : Use the same format as specified by GIC in
+  Documentation/devicetree/bindings/arm/gic.txt
+- interrupt-parent: phandle of irq parent for sysirq. The parent must
+  use the same interrupt-cells format as GIC.
+- reg: Physical base address of the intpol registers and length of memory
+  mapped region.
+
+Example:
+	sysirq: interrupt-controller@10200100 {
+		compatible = "mediatek,mt6589-sysirq", "mediatek,mt6577-sysirq";
+		interrupt-controller;
+		#interrupt-cells = <3>;
+		interrupt-parent = <&gic>;
+		reg = <0 0x10200100 0 0x1c>;
+	};