arm: mvebu: helios4: Enable CONFIG_BLK and CONFIG_DM_MMC

arm: mvebu: helios4: Enable CONFIG_BLK and CONFIG_DM_MMC

This patch enables CONFIG_BLK and CONFIG_DM_MMC on helios4 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.
====================================================

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Dennis Gilmore <dgilmore@redhat.com>