mmc: mxs-mmc: Drop unused includes

The MXS-MMC driver uses slot GPIO helpers and does not make
any use of <linux/gpio.h> or <linux/of_gpio.h> so drop these
surplus includes.

Cc: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>