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/p2771-0000-000_defconfig b/configs/p2771-0000-000_defconfig
index 4c4606a..a48ba26 100644
--- a/configs/p2771-0000-000_defconfig
+++ b/configs/p2771-0000-000_defconfig
@@ -25,6 +25,7 @@
 CONFIG_CMD_EXT4_WRITE=y
 CONFIG_CMD_FAT=y
 CONFIG_CMD_FS_GENERIC=y
+# CONFIG_BLK is not set
 CONFIG_TEGRA186_BPMP_I2C=y
 CONFIG_E1000=y
 CONFIG_RTL8169=y