ppc: Move CONFIG_MAX_MEM_MAPPED to common config.h

Moved CONFIG_MAX_MEM_MAPPED to the asm/config.h so its kept consistent
between the two current users (lib_ppc/board.c, 44x SPD DDR2).

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Stefan Roese <sr@denx.de>