hwrng: bcm2835 - Manage an optional clock

One of the last steps before bcm63xx-rng can be eliminated is to manage
a clock during hwrng::init and hwrng::cleanup, so fetch it in the probe
function, and manage it during these two steps when valid.

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>