configs: aspeed: Unset CONFIG_MMC

The aspeed board does not have an upstream MMC driver.

As CONFIG_MMC defaults on, the board would fail to build due to the
CONFIG_DM_MMC migration:

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

Change the defconfig to disable MMC until a driver is submitted.

Signed-off-by: Joel Stanley <joel@jms.id.au>