mmc: sdhci: merge two sdhci-pltfm.h into one

The structure sdhci_pltfm_data is not necessarily to be in a public
header like include/linux/mmc/sdhci-pltfm.h, so the patch moves it
into drivers/mmc/host/sdhci-pltfm.h and eliminates the former one.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Grant Likely <grant.likely@secretlab.ca>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>