ppc4xx: DDR/ECC: Use correct macros to clear error status

Use the correct macro instead of the hardcoded 0x4c to clear the ECC
status in the 440/460 DDR(2) error status register after ECC
initialization.

Also the non-440 parts (405EX(r) right now) and the IBM DDR PPC variants
(440GX) use a different registers to clear this error status. Use the
correct ones.

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