watchdog: xilinx: Add missing binding

Xilinx AXI/PLB soft-core watchdog Device Tree Bindings

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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>;

} ;