hwrng: iproc-rng200 - Add device tree bindings

Documents the IPROC random number generator device tree bindings
used in some Broadcom chipsets.

Reviewed-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>