HWRNG support for the timeriomem_rng driver

Required properties:
- compatible : "timeriomem_rng"
- reg : base address to sample from
- period : wait time in microseconds to use between samples

N.B. currently 'reg' must be four bytes wide and aligned

Example:

hwrng@44 {
	#address-cells = <1>;
	#size-cells = <1>;
	compatible = "timeriomem_rng";
	reg = <0x44 0x04>;
	period = <1000000>;
};