dt: watchdog: Add DT binding documentation for jz4740 watchdog timer

Add binding for jz4740 watchdog timer. It is a simple watchdog timer.
Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

Documentation/devicetree/bindings/watchdog/ingenic,jz4740-wdt.txt

+Ingenic Watchdog Timer (WDT) Controller for JZ4740
+
+Required properties:
+compatible: "ingenic,jz4740-watchdog"
+reg: Register address and length for watchdog registers
+
+Example:
+
+watchdog: jz4740-watchdog@0x10002000 {
+	compatible = "ingenic,jz4740-watchdog";
+	reg = <0x10002000 0x100>;
+};