Binding for simple gpio gated clock.

This binding uses the common clock binding[1].

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

Required properties:
- compatible : shall be "gpio-gate-clock".
- #clock-cells : from common clock binding; shall be set to 0.
- enable-gpios : GPIO reference for enabling and disabling the clock.

Optional properties:
- clocks: Maximum of one parent clock is supported.

Example:
	clock {
		compatible = "gpio-gate-clock";
		clocks = <&parentclk>;
		#clock-cells = <0>;
		enable-gpios = <&gpio 1 GPIO_ACTIVE_HIGH>;
	};
