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

Signed-off-by: Rameshwar Prasad Sahu <[e-mail]>
Signed-off-by: Loc Ho <[e-mail]>
Cc: Jon Masters <[e-mail]>
Cc: Alessandro Zummo <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: Will Deacon <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/devicetree/bindings/rtc/xgene-rtc.txt

···
+* 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>;
+};