DM: DM_MMC migration is now mandatory for non-SPL

As it has been now two years past the migration deadline, it is required
to have migrated.  Remove the check from the Makefile and rework some of
the Kconfig logic slightly to get the functional dependencies of DM_MMC
/ BLK right in both the SPL and non-SPL case.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/thuban_defconfig b/configs/thuban_defconfig
index c54b91e..116673c 100644
--- a/configs/thuban_defconfig
+++ b/configs/thuban_defconfig
@@ -72,9 +72,11 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_SPL_DM=y
+# CONFIG_SPL_BLK is not set
 CONFIG_BOOTCOUNT_LIMIT=y
 CONFIG_BOOTCOUNT_ENV=y
 CONFIG_DFU_NAND=y
+# CONFIG_SPL_DM_MMC is not set
 CONFIG_MMC_OMAP_HS=y
 CONFIG_MTD=y
 CONFIG_MTD_RAW_NAND=y