ARM: docs: add documentation binding for meson watchdog

Signed-off-by: Carlo Caione <carlo@caione.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

Documentation/devicetree/bindings/watchdog/meson6-wdt.txt

+Meson SoCs Watchdog timer
+
+Required properties:
+
+- compatible : should be "amlogic,meson6-wdt"
+- reg : Specifies base physical address and size of the registers.
+
+Example:
+
+wdt: watchdog@c1109900 {
+	compatible = "amlogic,meson6-wdt";
+	reg = <0xc1109900 0x8>;
+};