Binding for the axi-clkgen clock generator

This binding uses the common clock binding[1].

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

Required properties:
- compatible : shall be "adi,axi-clkgen".
- #clock-cells : from common clock binding; Should always be set to 0.
- reg : Address and length of the axi-clkgen register set.
- clocks : Phandle and clock specifier for the parent clock.

Optional properties:
- clock-output-names : From common clock binding.

Example:
	clock@0xff000000 {
		compatible = "adi,axi-clkgen";
		#clock-cells = <0>;
		reg = <0xff000000 0x1000>;
		clocks = <&osc 1>;
	};