Add binding documentation for Zodiac Watchdog Timer

This patchs adds documentation for the binding of the Zodiac RAVE
Switch Watchdog Processor. This is an i2c based watchdog.

Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

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

+Zodiac RAVE Watchdog Timer
+
+Required properties:
+- compatible: must be "zii,rave-wdt"
+- reg: i2c slave address of device, usually 0x38
+
+Optional Properties:
+- timeout-sec: Watchdog timeout value in seconds.
+- reset-duration-ms: Duration of the pulse generated when the watchdog times
+  out. Value in milliseconds.
+
+Example:
+
+	watchdog@38 {
+		compatible = "zii,rave-wdt";
+		reg = <0x38>;
+		timeout-sec = <30>;
+		reset-duration-ms = <30>;
+	};