mmc: sdhci-pxav2: Drop unused include

The SDHCI PXAv2 driver uses slot GPIO helpers and does not make
any use of <linux/gpio.h> so drop this surplus include.

Cc: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>