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

The patch turns the sdhci-of-core common stuff into helper functions
added into sdhci-pltfm.c, and makes sdhci-of device drviers self
registered using the same pair of .probe and .remove used by
sdhci-pltfm device drivers.

As a result, sdhci-of-core.c and sdhci-of.h can be eliminated with
those common things merged into sdhci-pltfm.c and sdhci-pltfm.h
respectively.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>