[PATCH] PPC4xx: Split 4xx SPD SDRAM init routines into 2 files

Since the existing 4xx SPD SDRAM initialization routines for the
405 SDRAM controller and the 440 DDR controller don't have much in
common this patch splits both drivers into different files.

This is in preparation for the 440 DDR2 controller support (440SP/e).

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