mmc: sdhci-pltfm: add -pltfm driver for imx35/51

This driver adds basic support for the esdhc-core found on e.g.
imx35/51, as a platform driver.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Tested-by: Eric Bénard <eric@eukrea.com>
Signed-off-by: Chris Ball <cjb@laptop.org>