dt-bindings: rng: Describe Exynos4 PRNG bindings

Document the bindings used by exynos-rng Pseudo Random Number Generator
driver.
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Documentation/devicetree/bindings/rng/samsung,exynos-rng4.txt

+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";
+	};