mmc: mxcmmc: Allow binding a regulator to manage the MMC card voltage

This implementation is based on the pxamci.c driver and it will
be used to support the mx31_3ds machine.

Signed-off-by: Alberto Panizzo <maramaopercheseimorto@gmail.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>