mmc: mxcmmc: Drop unused includes

The MXCMMC 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: Jun Qian <hangdianqj@163.com>
Cc: Matteo Facchinetti <matteo.facchinetti@sirius-es.it>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>