hwrng: bcm2835 - Define a driver private context

Instead of making hwrng::priv host the base register address, define a
driver private context, make it per platform device instance and pass it
down the different functions.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>