mmc: sdhci: make sdhci-pltfm device drivers self registered

The patch turns the common stuff in sdhci-pltfm.c into functions, and
add device drivers their own .probe and .remove which in turn call
into the common functions, so that those sdhci-pltfm device drivers
register itself and keep all device specific things away from common
sdhci-pltfm file.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>