Binding for Xilinx pl clocks

This binding uses the common clock binding[1].

[1] Documentation/devicetree/bindings/clock/clock-bindings.txt

Required properties:
 - compatible: Must be 'xlnx,fclk'
 - clocks: Handle to input clock

Example:
	fclk3: fclk3 {
		status = "disabled";
		compatible = "xlnx,fclk";
		clocks = <&clkc 71>;
	};
