Documentation/devicetree/bindings: add documentation for the APM X-Gene SoC RTC DTS binding

* APM X-Gene Real Time Clock

RTC controller for the APM X-Gene Real Time Clock

Required properties:

- compatible : Should be "apm,xgene-rtc"

- reg: physical base address of the controller and length of memory mapped

  region.

- interrupts: IRQ line for the RTC.

- #clock-cells: Should be 1.

- clocks: Reference to the clock entry.

Example:

rtcclk: rtcclk {

	compatible = "fixed-clock";

	#clock-cells = <1>;

	clock-frequency = <100000000>;

	clock-output-names = "rtcclk";

};

rtc: rtc@10510000 {

	compatible = "apm,xgene-rtc";

	reg = <0x0 0x10510000 0x0 0x400>;

	interrupts = <0x0 0x46 0x4>;

	#clock-cells = <1>;

	clocks = <&rtcclk 0>;

};