board: tbs2910: Convert mmc to driver model

So we don't need to duplicate settings from the device tree in the board file.

Signed-off-by: Soeren Moch <smoch@web.de>
diff --git a/configs/tbs2910_defconfig b/configs/tbs2910_defconfig
index 54e34df..4d755c8 100644
--- a/configs/tbs2910_defconfig
+++ b/configs/tbs2910_defconfig
@@ -46,6 +46,7 @@
 CONFIG_DM_I2C=y
 CONFIG_SYS_I2C_MXC=y
 CONFIG_DM_KEYBOARD=y
+CONFIG_DM_MMC=y
 CONFIG_FSL_ESDHC=y
 CONFIG_PHYLIB=y
 CONFIG_MII=y