arm: mvebu: ds412: Enable CONFIG_BLK

arm: mvebu: ds412: Enable CONFIG_BLK

This patch enables CONFIG_BLK for the ds414 Armada XP based platform.
This removes this build 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.
====================================================

Please note that I don't have this board and can't test this change.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Phil Sutter <phil@nwl.cc>