watchdog: lantiq: add device tree binding documentation

The binding was not documented before, add the documentation now.

Signed-off-by: Hauke Mehrtens <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Patchwork: https://patchwork.linux-mips.org/patch/17119/
Signed-off-by: Ralf Baechle <[e-mail]>

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

···
+Lantiq WTD watchdog binding
+============================
+
+This describes the binding of the Lantiq watchdog driver.
+
+-------------------------------------------------------------------------------
+Required properties:
+- compatible		: Should be one of
+				"lantiq,wdt"
+				"lantiq,xrx100-wdt"
+				"lantiq,xrx200-wdt", "lantiq,xrx100-wdt"
+				"lantiq,falcon-wdt"
+- reg			: Address of the watchdog block
+- lantiq,rcu		: A phandle to the RCU syscon (required for
+			  "lantiq,falcon-wdt" and "lantiq,xrx100-wdt")
+
+-------------------------------------------------------------------------------
+Example for the watchdog on the xRX200 SoCs:
+		watchdog@803f0 {
+			compatible = "lantiq,xrx200-wdt", "lantiq,xrx100-wdt";
+			reg = <0x803f0 0x10>;
+
+			lantiq,rcu = <&rcu0>;
+		};