hwrng: imx-rngc - use devres for hwrng registration

Replace hwrng_register with devm_hwrng_register and let devres unregister
our hwrng when the device is removed.

It's possible to do this now that devres also handles clock
disable+uprepare. When we had to disable+unprepare the clock ourselves,
we had to unregister the hwrng before this and couldn't use devres.

There's nothing left to do for imx_rngc_remove, this function can go.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>