dt-bindings: rng: add clock to DT binding documentation for hwrng

- compatible : should be "amlogic,meson-rng"

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

Optional properties:

- clocks : phandle to the following named clocks

- clock-names: Name of core clock, must be "core"

Example:

rng {

	compatible = "amlogic,meson-rng";

	reg = <0x0 0xc8834000 0x0 0x4>;

	clocks = <&clkc CLKID_RNG0>;

	clock-names = "core";

};