mmc: mxmmc: include linux/highmem.h

The highmem conversion caused a build error in some configurations:

drivers/mmc/host/mxcmmc.c: In function 'mxcmci_transfer_data':
drivers/mmc/host/mxcmmc.c:622:10: error: implicit declaration of function 'kmap_atomic'; did you mean 'in_atomic'? [-Werror=implicit-function-declaration]

This includes the correct header file.

Fixes: b189e758 ("mmc: mxcmmc: handle highmem pages")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>