Binding for Keystone gate control driver which uses PSC controller IP.

This binding uses the common clock binding[1].

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

Required properties:
- compatible : shall be "ti,keystone,psc-clock".
- #clock-cells : from common clock binding; shall be set to 0.
- clocks : parent clock phandle
- reg :	psc control and domain address address space
- reg-names : psc control and domain registers
- domain-id : psc domain id needed to check the transition state register

Optional properties:
- clock-output-names : From common clock binding to override the
			default output clock name
Example:
	clkusb: clkusb {
		#clock-cells = <0>;
		compatible = "ti,keystone,psc-clock";
		clocks = <&chipclk16>;
		clock-output-names = "usb";
		reg = <0x02350008 0xb00>, <0x02350000 0x400>;
		reg-names = "control", "domain";
		domain-id = <0>;
	};