dt-bindings: rng: Northstar Plus SoC rng bindings

Document the bindings used by Northstar Plus(NSP) SoC random number
generator.

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>