mx28evk: fix build error

Fix:
drivers/mmc/libmmc.o: In function `mxsmmc_send_cmd':
/home/ag/git/u-boot/drivers/mmc/mxsmmc.c:210: undefined reference to `mxs_dma_desc_append'
/home/ag/git/u-boot/drivers/mmc/mxsmmc.c:211: undefined reference to `mxs_dma_go'
drivers/mmc/libmmc.o: In function `mxsmmc_initialize':
/home/ag/git/u-boot/drivers/mmc/mxsmmc.c:306: undefined reference to `mxs_dma_desc_alloc'

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Fabio Estevam <fabio.estevam@freescale.com>