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/drivers/block/Kconfig b/drivers/block/Kconfig
index 80eea84..fe5aa07 100644
--- a/drivers/block/Kconfig
+++ b/drivers/block/Kconfig
@@ -1,6 +1,7 @@
 config BLK
 	bool "Support block devices"
 	depends on DM
+	default y if DM_MMC
 	help
 	  Enable support for block devices, such as SCSI, MMC and USB
 	  flash sticks. These provide a block-level interface which permits