hwrng: bcm2835 - Abstract I/O accessors

In preparation for allowing BCM63xx to use this driver, we abstract I/O
accessors such that we can easily change those later on.

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>