Binding for simple gpio clock multiplexer.

This binding uses the common clock binding[1].

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

Required properties:
- compatible : shall be "gpio-mux-clock".
- clocks: list of two references to parent clocks.
- #clock-cells : from common clock binding; shall be set to 0.
- select-gpios : GPIO reference for selecting the parent clock.

Example:
	clock {
		compatible = "gpio-mux-clock";
		clocks = <&parentclk1>, <&parentclk2>;
		#clock-cells = <0>;
		select-gpios = <&gpio 1 GPIO_ACTIVE_HIGH>;
	};