dt-bindings: watchdog: Add Aspeed watchdog timer bindings

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

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

+Aspeed Watchdog Timer
+
+Required properties:
+ - compatible: must be one of:
+	- "aspeed,ast2400-wdt"
+	- "aspeed,ast2500-wdt"
+
+ - reg: physical base address of the controller and length of memory mapped
+   region
+
+Example:
+
+	wdt1: watchdog@1e785000 {
+		compatible = "aspeed,ast2400-wdt";
+		reg = <0x1e785000 0x1c>;
+	};