Documentation: rng: Add X-Gene SoC RNG driver documentation

Add X-Gene SoC RNG driver documentation.
Signed-off-by: NFeng Kan <fkan@apm.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

Documentation/devicetree/bindings/rng/apm,rng.txt

+APM X-Gene SoC random number generator.
+
+Required properties:
+
+- compatible  : should be "apm,xgene-rng"
+- reg         : specifies base physical address and size of the registers map
+- clocks      : phandle to clock-controller plus clock-specifier pair
+- interrupts  : specify the fault interrupt for the RNG device
+
+Example:
+
+	rng: rng@10520000 {
+		compatible = "apm,xgene-rng";
+		reg = <0x0 0x10520000 0x0 0x100>;
+		interrupts =   <0x0 0x41 0x4>;
+		clocks = <&rngpkaclk 0>;
+	};