MPC86xx: set CONFIG_MAX_MEM_MAPPED to 2G by default

Currently, we get 256MB as the default, but since all the 86xx
board configs define a 2G BAT mapping for RAM, raise default
to 2G.

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Acked-by: Jon Loeliger <jdl@freescale.com>