powerpc/8xx: move setup_initial_memory_limit() into 8xx_mmu.c

Now we have a 8xx specific .c file for that so put it in there
as other powerpc variants do

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <oss@buserror.net>