ppc4xx: DDR2: Complete RDSS configuration on non-SPD based boards

As described in item #10 of the SDRAM initialization (chapter 22.2.9
of the PPC460EX/EXr/GT users manual), RDSS may need to be adjusted. The
code for this is now factored out and executed for non-SPD based boards
as well.

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