hwrng: brcm63xx - document device tree bindings

BCM6368 Random number generator

Required properties:

- compatible : should be "brcm,bcm6368-rng"

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

- clocks : phandle to clock-controller plus clock-specifier pair

- clock-names : "ipsec" as a clock name

Example:

	random: rng@10004180 {

		compatible = "brcm,bcm6368-rng";

		reg = <0x10004180 0x14>;

		clocks = <&periph_clk 18>;

		clock-names = "ipsec";

	};