[POWERPC] 8xx: Set initial memory limit.

The 8xx can only support a max of 8M during early boot (it seems a lot of
8xx boards only have 8M so the bug was never triggered), but the early
allocator isn't aware of this.  The following change makes it able to run
with larger memory.

Signed-off-by: John Traill <john.traill@freescale.com>
Signed-off-by: Vitaly Bordug <vitb@kernel.crashing.org>
Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>