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>

include/configs/mx28evk.h

@@ -126,6 +126,11 @@
 #define CONFIG_SYS_BAUDRATE_TABLE	{ 9600, 19200, 38400, 57600, 115200 }
 
 /*
+ * DMA
+ */
+#define CONFIG_APBH_DMA
+
+/*
  * MMC Driver
  */
 #define CONFIG_ENV_IS_IN_MMC