dkb: make mmc command as default enabled

Signed-off-by: Lei Wen <leiwen@marvell.com>

include/configs/dkb.h

@@ -47,6 +47,7 @@
 #define CONFIG_SYS_NO_FLASH		/* Declare no flash (NOR/SPI) */
 #include <config_cmd_default.h>
 #define CONFIG_CMD_I2C
+#define CONFIG_CMD_MMC
 #undef CONFIG_CMD_NET
 #undef CONFIG_CMD_NFS
 /*