hwrng: bcm2835 - Use device managed helpers

Now that we have moved the RNG disabling into a hwrng::cleanup callback,
we can use the device managed registration operation and remove our
remove callback since it won't do anything necessary.

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>