dt-bindings: rng: Document BCM7278 RNG200 compatible

BCM7278 includes a RGN200 hardware random number generator, document the
compatible string for that version of the IP.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>