mmc: sdhci-pltfm: Convert to use the SET_SYSTEM_SLEEP_PM_OPS

Move the system PM callbacks within #ifdef CONFIG_PM_SLEEP as to avoid
them being build when not used. This also allows us to use the
SET_SYSTEM_SLEEP_PM_OPS macro which simplifies the code.

Within this context it also makes sense to move the declaration of the
struct sdhci_pltfm_pmops, outside the #ifdef CONFIG_PM as the
SET_SYSTEM_SLEEP_PM_OPS deals with this. This further simplifies the code.

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