mmc: mxcmmc: explicitly includes mach/hardware.h

When indirect inclusion to <mach/hardware.h> via <mach/gpio.h> gets
removed, we will see the following compile error.

  CC      drivers/mmc/host/mxcmmc.o
drivers/mmc/host/mxcmmc.c: In function ‘mxcmci_init_card’:
drivers/mmc/host/mxcmmc.c:811:2: error: implicit declaration of function ‘cpu_is_mx3’

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>