hwrng: iproc-rng200 - Add Broadcom IPROC RNG driver

This adds a driver for random number generator present on Broadcom
IPROC devices.

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>