ppc4xx: Merge PPC4xx DDR and DDR2 ECC handling

This patch merges the ECC handling (ECC parity byte writing) into one
file (ecc.c) for all PPC4xx SDRAM controllers except for PPC440EPx/GRx.
This exception is because only those PPC's use the completely different
Denali SDRAM controller core.

Previously we had two routines to generate/write the ECC parity bytes.
With this patch we now only have one core function left.

Tested on Kilauea (no ECC) and Katmai (with and without ECC).

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Felix Radensky <felix@embedded-sol.com>
Cc: Grant Erickson <gerickson@nuovations.com>
Cc: Pieter Voorthuijsen <pv@prodrive.nl>