devicetree: Add Cadence WDT devicetree bindings documentation

Add cadence-wdt bindings documentation.
Signed-off-by: Harini Katakam <harinik@xilinx.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

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

+Zynq Watchdog Device Tree Bindings
+-------------------------------------------
+
+Required properties:
+- compatible		: Should be "cdns,wdt-r1p2".
+- clocks		: This is pclk (APB clock).
+- interrupts		: This is wd_irq - watchdog timeout interrupt.
+- interrupt-parent	: Must be core interrupt controller.
+
+Optional properties
+- reset-on-timeout	: If this property exists, then a reset is done
+			  when watchdog times out.
+- timeout-sec		: Watchdog timeout value (in seconds).
+
+Example:
+	watchdog@f8005000 {
+		compatible = "cdns,wdt-r1p2";
+		clocks = <&clkc 45>;
+		interrupt-parent = <&intc>;
+		interrupts = <0 9 1>;
+		reg = <0xf8005000 0x1000>;
+		reset-on-timeout;
+		timeout-sec = <10>;
+	};