HWRNG support for the n2_rng driver

Required properties:
- reg		: base address to sample from
- compatible	: should contain one of the following
	RNG versions:
	- 'SUNW,n2-rng' for Niagara 2 Platform (SUN UltraSPARC T2 CPU)
	- 'SUNW,vf-rng' for Victoria Falls Platform (SUN UltraSPARC T2 Plus CPU)
	- 'SUNW,kt-rng' for Rainbow/Yosemite Falls Platform (SUN SPARC T3/T4), (UltraSPARC KT/Niagara 3 - development names)
	more recent systems (after Oracle acquisition of SUN)
	- 'ORCL,m4-rng' for SPARC T5/M5
	- 'ORCL,m7-rng' for SPARC T7/M7

Examples:
/* linux LDOM on SPARC T5-2 */
Node 0xf029a4f4
	.node:  f029a4f4
	rng-#units:  00000002
	compatible: 'ORCL,m4-rng'
	reg:  0000000e
	name: 'random-number-generator'

/* solaris on SPARC M7-8 */
Node 0xf028c08c
	rng-#units:  00000003
	compatible: 'ORCL,m7-rng'
	reg:  0000000e
	name:  'random-number-generator'

PS: see as well prtconfs.git by DaveM
