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

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>;

	};