hwrng: bcm63xx - add device tree support

Adds device tree support for BCM6368, which seems to be the only BCM63xx with
bcm63xx-rng support.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>