arm: mvebu: db-88f6720: Enable CONFIG_BLK

This patch enables CONFIG_BLK to remove this compile warning:

===================== WARNING ======================
This board does not use CONFIG_DM_USB. Please update
the board to use CONFIG_DM_USB before the v2019.07 release.
Failure to update by the deadline may result in board removal.
See doc/driver-model/MIGRATION.txt for more info.
====================================================
Signed-off-by: NStefan Roese <sr@denx.de>

configs/db-88f6720_defconfig

@@ -44,6 +44,7 @@ CONFIG_USE_ENV_SPI_MAX_HZ=y
 CONFIG_ENV_SPI_MAX_HZ=50000000
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_SPL_OF_TRANSLATE=y
+CONFIG_BLK=y
 CONFIG_MISC=y
 # CONFIG_MMC is not set
 CONFIG_SPI_FLASH=y