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_evm_defconfig b/configs/am335x_evm_defconfig
index a2cfef3..b9f1051 100644
--- a/configs/am335x_evm_defconfig
+++ b/configs/am335x_evm_defconfig
@@ -34,6 +34,7 @@
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_OF_CONTROL=y
 CONFIG_OF_LIST="am335x-evm am335x-bone am335x-boneblack am335x-evmsk am335x-bonegreen am335x-icev2"
+# CONFIG_BLK is not set
 CONFIG_DFU_MMC=y
 CONFIG_DFU_NAND=y
 CONFIG_DFU_RAM=y