arm: mvebu: db-88f6820-gp: Enable BLK and DM support

arm: mvebu: db-88f6820-gp: Enable BLK and DM support

This patch enables CONFIG_BLK and some DM enabled drivers on
db-88f6820-gp to remove these compile warnings:

===================== WARNING ======================
This board does not use CONFIG_DM_MMC. Please update
the board to use CONFIG_DM_MMC before the v2019.04 release.
Failure to update by the deadline may result in board removal.
See doc/driver-model/MIGRATION.txt for more info.
====================================================
===================== 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.
====================================================
===================== WARNING ======================
This board does use CONFIG_LIBATA but has CONFIG_AHCI not
enabled. Please update the storage controller driver to use
CONFIG_AHCI 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: Stefan Roese <sr@denx.de>
Reviewed-by: Chris Packham <judge.packham@gmail.com>