dt-bindings: hwrng: Add Samsung Exynos 5250+ True RNG bindings

Add binding documentation for the True Random Number Generator
found on Samsung Exynos 5250+ SoCs.
Acked-by: NRob Herring <robh@kernel.org>
Reviewed-by: NKrzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: NŁukasz Stelmach <l.stelmach@samsung.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

Documentation/devicetree/bindings/rng/samsung,exynos5250-trng.txt

+Exynos True Random Number Generator
+
+Required properties:
+
+- compatible  : Should be "samsung,exynos5250-trng".
+- 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@10830600 {
+		compatible = "samsung,exynos5250-trng";
+		reg = <0x10830600 0x100>;
+		clocks = <&clock CLK_SSS>;
+		clock-names = "secss";
+	};