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/k2g_evm_defconfig b/configs/k2g_evm_defconfig
index 13b91cd..27659ba 100644
--- a/configs/k2g_evm_defconfig
+++ b/configs/k2g_evm_defconfig
@@ -35,6 +35,7 @@
 CONFIG_CMD_UBI=y
 CONFIG_OF_CONTROL=y
 CONFIG_DM=y
+# CONFIG_BLK is not set
 CONFIG_DM_MMC=y
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH=y