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/am335x_boneblack_vboot_defconfig b/configs/am335x_boneblack_vboot_defconfig
index e0110d1..68769ea 100644
--- a/configs/am335x_boneblack_vboot_defconfig
+++ b/configs/am335x_boneblack_vboot_defconfig
@@ -38,6 +38,7 @@
 CONFIG_CMD_FAT=y
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_OF_CONTROL=y
+# CONFIG_BLK is not set
 CONFIG_DFU_MMC=y
 CONFIG_DFU_RAM=y
 CONFIG_DM_I2C=y