arm: imx7s-warp: Convert to DM MMC initialization

Converts from fixed initialization of MMC to DM initialization of MMC.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
diff --git a/configs/warp7_defconfig b/configs/warp7_defconfig
index 66533de..ed2a9ff 100644
--- a/configs/warp7_defconfig
+++ b/configs/warp7_defconfig
@@ -34,6 +34,7 @@
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_DFU_MMC=y
+CONFIG_DM_MMC=y
 CONFIG_FSL_ESDHC=y
 CONFIG_OPTEE=y
 CONFIG_USB=y