mmc: wmt-sdmmc: Drop unused include

The WMT SDMMC driver uses slot GPIO helpers and does not make
any use of <linux/gpio.h> so drop this surplus include.

Cc: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>