dt-bindings: rng: Describe Exynos4 PRNG bindings

Exynos Pseudo Random Number Generator

Required properties:

- compatible  : Should be "samsung,exynos4-rng".

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

- clocks      : Phandle to clock-controller plus clock-specifier pair.

- clock-names : "secss" as a clock name.

Example:

	rng@10830400 {

		compatible = "samsung,exynos4-rng";

		reg = <0x10830400 0x200>;

		clocks = <&clock CLK_SSS>;

		clock-names = "secss";

	};