ARM: DT: msm: Add Qualcomm's PRNG driver binding document

Qualcomm MSM pseudo random number generator.

Required properties:

- compatible  : should be "qcom,prng"

- reg         : specifies base physical address and size of the registers map

- clocks      : phandle to clock-controller plus clock-specifier pair

- clock-names : "core" clocks all registers, FIFO and circuits in PRNG IP block

Example:

	rng@f9bff000 {

		compatible = "qcom,prng";

		reg = <0xf9bff000 0x200>;

		clocks = <&clock GCC_PRNG_AHB_CLK>;

		clock-names = "core";

	};