mmc: sdhci-pltfm: export sdhci_pltfm_suspend/resume

This will be useful when drivers want to reuse either suspend or
resume callback instead of whole of sdhci_pltfm_pmops.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>