5xxx, fdt: move fdt_fixup_memory() to cpu.c file

5xxx, fdt: move fdt_fixup_memory() to cpu.c file

u-boot updates, before starting Linux, the memory node in the
DTS. As this is a "standard" feature, move this functionality
to the cpu.c file for mpc5xxx and mpc512x processors.

Signed-off-by: Heiko Schocher <hs@denx.de>