dt-bindings: watchdog: add description for UniPhier WDT controller

Add uniphier-wdt dt-bindings documentation.
Signed-off-by: NKeiji Hayashibara <hayashibara.keiji@socionext.com>
Acked-by: NRob Herring <robh@kernel.org>
Reviewed-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NWim Van Sebroeck <wim@iguana.be>

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

+UniPhier watchdog timer controller
+
+This UniPhier watchdog timer controller must be under sysctrl node.
+
+Required properties:
+- compatible: should be "socionext,uniphier-wdt"
+
+Example:
+
+	sysctrl@61840000 {
+		compatible = "socionext,uniphier-ld11-sysctrl",
+			     "simple-mfd", "syscon";
+		reg = <0x61840000 0x4000>;
+
+		watchdog {
+			compatible = "socionext,uniphier-wdt";
+		}
+
+		other nodes ...
+	};