watchdog: bcm7038: add device tree binding documentation

Add device tree binding documentation for the watchdog hardware block
on bcm7038 and newer SoCs.
Signed-off-by: Justin Chen <justinpopo6@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

Documentation/devicetree/bindings/watchdog/brcm,bcm7038-wdt.txt

+BCM7038 Watchdog timer
+
+Required properties:
+
+- compatible : should be "brcm,bcm7038-wdt"
+- reg : Specifies base physical address and size of the registers.
+
+Optional properties:
+
+- clocks: The clock running the watchdog. If no clock is found the
+	  driver will default to 27000000 Hz.
+
+Example:
+
+watchdog@f040a7e8 {
+	compatible = "brcm,bcm7038-wdt";
+	clocks = <&upg_fixed>;
+	reg = <0xf040a7e8 0x16>;
+};