hwrng: imx-rngc - add runtime pm

Add runtime power management to the imx-rngc driver. Disable the
peripheral clock when the rngc is idle.

The callback functions from struct hwrng wake the rngc up when they're
called and set it to idle on exit. Helper functions which are invoked
from the callbacks assume that the rngc is active.

Device init and probe are done before runtime pm is enabled. The
peripheral clock will be handled manually during these steps. Do not use
devres any more to enable/disable the peripheral clock, this conflicts
with runtime pm.

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