mmc: sdhci-pltfm: export pltfm suspend/resume api

It is helpful for platform code to use to eliminate duplicated code.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>