hwrng: bcm2835 - Enable BCM2835 RNG to work on BCM63xx platforms

We have now incorporated all necessary functionality for the BCM63xx
platforms to successfully migrate over bcm2835-rng, so add the final
bits: Kconfig selection and proper platform_device device type matching
to keep the same platform device name for registration to work.

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