Binding for simple fixed-rate clock sources.

This binding uses the common clock binding[1].

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

Required properties:
- compatible : shall be "fixed-clock".
- #clock-cells : from common clock binding; shall be set to 0.
- clock-frequency : frequency of clock in Hz. Should be a single cell.

Optional properties:
- gpios : From common gpio binding; gpio connection to clock enable pin.
- clock-output-names : From common clock binding.

Example:
	clock {
		compatible = "fixed-clock";
		#clock-cells = <0>;
		clock-frequency = <1000000000>;
	};