dm: blk: Enable CONFIG_BLK if DM_MMC is enabled

To speed up conversion to CONFIG_BLK, enable it by default when DM_MMC is
enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/ventana_defconfig b/configs/ventana_defconfig
index 56e7ba3..e247f9a 100644
--- a/configs/ventana_defconfig
+++ b/configs/ventana_defconfig
@@ -27,6 +27,7 @@
 CONFIG_CMD_FAT=y
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_SPL_DM=y
+# CONFIG_BLK is not set
 CONFIG_DM_PMIC=y
 CONFIG_DM_REGULATOR=y
 CONFIG_DM_REGULATOR_FIXED=y