ppc4xx: PMC440: Only use one CS (rank) in DDR2 configuration

This patch fixes a problem spotted by Mikhail Zolotaryov on Sequoia with
the DDR2 configuration to only use one CS (rank). As this code is most
likely copied from the original Sequoia version, this error was copied
as well.

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