hwrng: bcm2835 - Support Broadcom NSP SoC rng

This supports the random number generator available in NSP SoC.
Masks the rng interrupt for NSP.

Signed-off-by: Yendapally Reddy Dhananjaya Reddy <yendapally.reddy@broadcom.com>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>