sbc8641d: fix LAW so board doesn't hang on DDR init

All versions between now and since this commit:

  commit bd76729b
  MPC86xx: set CONFIG_MAX_MEM_MAPPED to 2G by default

will fail to allow the SBC8641D to get past DDR init, because the
LAW config was overlapping.  Eventually this board will do SPD
EEPROM config, but for now this gets the board working again.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>