dt-bindings: rng: Add DT binding entry for msm-rng

* RNG (Random Number Generator)

Required properties:

- compatible : Should be "qcom,msm-rng"

- reg        : Offset and length of the register set for the device

Optional property:

- qcom,msm-rng-iface-clk : If the device uses iface-clk.

- qcom,no-qrng-config    : Flag to decide whether the driver do the hardware configuration or not.

Example:

	qcom,msm-rng@f9bff000 {

		compatible = "qcom,msm-rng";

		reg = <0xf9bff000 0x200>;

		qcom,msm-rng-iface-clk;

		qcom,no-qrng-config;

	};