watchdog: xilinx: Add missing binding

Document current driver binding.
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

Documentation/devicetree/bindings/watchdog/of-xilinx-wdt.txt

+Xilinx AXI/PLB soft-core watchdog Device Tree Bindings
+---------------------------------------------------------
+
+Required properties:
+- compatible		: Should be "xlnx,xps-timebase-wdt-1.00.a" or
+			  "xlnx,xps-timebase-wdt-1.01.a".
+- reg			: Physical base address and size
+
+Optional properties:
+- clock-frequency	: Frequency of clock in Hz
+- xlnx,wdt-enable-once	: 0 - Watchdog can be restarted
+			  1 - Watchdog can be enabled just once
+- xlnx,wdt-interval	: Watchdog timeout interval in 2^<val> clock cycles,
+			  <val> is integer from 8 to 31.
+
+Example:
+axi-timebase-wdt@40100000 {
+	clock-frequency = <50000000>;
+	compatible = "xlnx,xps-timebase-wdt-1.00.a";
+	reg = <0x40100000 0x10000>;
+	xlnx,wdt-enable-once = <0x0>;
+	xlnx,wdt-interval = <0x1b>;
+} ;