hwrng: imx-rngc - simplify the power management definitions

Use the SIMPLE_DEV_PM_OPS() macro instead of populating a struct
dev_pm_ops directly. The suspend and resume functions will now be used
for both hibernation and suspend to ram.

If power management is disabled, SIMPLE_DEV_PM_OPS() evaluates to
nothing, The two functions won't be used and won't be included in the
kernel. Mark them as __maybe_unused to clarify that this is intended
behaviour.

With these modifications in place, we don't need the #ifdefs for power
management any more.

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