watchdog: lantiq: add device tree binding documentation

The binding was not documented before, add the documentation now.
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Rob Herring <robh@kernel.org>
Cc: martin.blumenstingl@googlemail.com
Cc: john@phrozen.org
Cc: andy.shevchenko@gmail.com
Cc: p.zabel@pengutronix.de
Cc: kishon@ti.com
Cc: mark.rutland@arm.com
Cc: linux-mips@linux-mips.org
Cc: linux-mtd@lists.infradead.org
Cc: linux-watchdog@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: linux-spi@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/17119/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

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>;
+		};