ppc4xx: Fix common ECC generation code for 440GP style platforms

This patch makes the common 4xx ECC code really usable on 440GP style
platforms.

Since the IBM DDR controller used on 440GP/GX/EP/GR is not register
compatible to the IBM DDR/2 controller used on 405EX/440SP/SPe/460EX/GT
we need to make some processor dependant defines used later on by the
driver.

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