hwrng: bcm2835 - Move enabling to hwrng::init

We should be moving the enabling of the HWRNG into a hwrng::init
callback since we can be disabled and enabled every time a different
hwrng is selected in the system.

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