ppc4xx: Fix problem with CONFIG_MAX_MEM_MAPPED in include/asm-ppc/config.h

CONFIG_SDRAM_PPC4xx_IBM_DDR2 is not set when include/asm-ppc/config.h is
included. So for katmai, CONFIG_MAX_MEM_MAPPED will get set to 256MB.

It makes perfect sense to set CONFIG_MAX_MEM_MAPPED to 2GB for all PPC4xx
boards right now.

Signed-off-by: Stefan Roese <sr@denx.de>