ARM: mediatek: dts: Add bindings for watchdog

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

Documentation/devicetree/bindings/watchdog/mtk-wdt.txt

+Mediatek SoCs Watchdog timer
+
+Required properties:
+
+- compatible : should be "mediatek,mt6589-wdt"
+- reg : Specifies base physical address and size of the registers.
+
+Example:
+
+wdt: watchdog@010000000 {
+	compatible = "mediatek,mt6589-wdt";
+	reg = <0x10000000 0x18>;
+};