dt-bindings: rng: add bindings doc for Keystone SA HWRNG driver

Keystone SoC Hardware Random Number Generator(HWRNG) Module

On Keystone SoCs HWRNG module is a submodule of the Security Accelerator.

- compatible: should be "ti,keystone-rng"

- ti,syscon-sa-cfg: phandle to syscon node of the SA configuration registers.

		    This registers are shared between hwrng and crypto drivers.

- clocks: phandle to the reference clocks for the subsystem

- clock-names: functional clock name. Should be set to "fck"

- reg: HWRNG module register space

Example:

/* K2HK */

rng@24000 {

	compatible = "ti,keystone-rng";

	ti,syscon-sa-cfg = <&sa_config>;

	clocks = <&clksa>;

	clock-names = "fck";

	reg = <0x24000 0x1000>;

};