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

This adds Qualcomm PRNG driver device tree binding documentation
to use as an example in dts trees.
Signed-off-by: NStanimir Varbanov <svarbanov@mm-sol.com>
Reviewed-by: NStephen Boyd <sboyd@codeaurora.org>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

Documentation/devicetree/bindings/rng/qcom,prng.txt

+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";
+	};