ppc4xx: Fix incorrect MODTx setup for some DIMM configurations

This patch fixes a problem with incorrect MODTx (On Die Termination)
setup for a configuration with multiple DIMM's and multiple ranks.
Without this change Katmai was unable to boot Linux with DDR2 frequency
>= 533MHz and mem>=3GB. With this patch Katmai successfully boots Linux
with DDR2 frequency = 640MHz and mem=4GB.

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