mmc: core: Refactor code to register the MMC PM notifier

Instead of checking for "#ifdef" directly in the code, let's invent a pair
of mmc core functions to deal with register/unregister the MMC PM notifier
block. Implement stubs for these functions when CONFIG_PM_SLEEP is unset,
as in that case the PM notifiers isn't used.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>