arm: omap2: am43xx: Enable CONFIG_BLK

With DM_MMC enabled, enable CONFIG_BLK to remove this
compile warning for am43xx based targets:

===================== 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.
====================================================

Targets were compile tested, build was clean.

Signed-off-by: Suniel Mahesh <sunil.m@techveda.org>