mmc: mxcmmc: Use mmc_regulator_get_supply() API

This patch replaces regulator manipulation with
mmc_regulator_get_supply() function from MMC core.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>