mmc: sdhci-pltfm: add suspend/resume functions

This patch adds the suspend and resume functions
in the sdhci-pltfm device driver.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>