ppc4xx: Canyonlands-NAND-boot: Support 2 Crucial 512MByte SODIMM's

Some Canyonlands boards are equipped with different SODIMM's. This is no
problem with the "normal" NOR booting Canyonlands U-Boot, since it
automatically detects the SODIMM's via SPD data and correctly configures
them. But the NAND booting version is different. Here we only have 4k
of image size to completely setup the hardware, including DDR2 setup.
So we need to use a fixed DDR2 setup here. This doesn't work for different
SODIMM's right now.

Currently only this Crucial SODIMM is support:
CT6464AC667.8FB (dual ranked)

Now some boards are shipped with this SODIMM:
CT6464AC667.4FE (single ranked)

This patch now supports both SODIMM's by configuring first for the dual
ranked DIMM. A quick shows, if this module is really installed. If this test
fails, the DDR2 controller is re-configured for the single
ranked SODIMM.

Tested with those SODIMM's:

CT6464AC667.8FB (dual ranked)
CT6464AC667.4FE (single ranked)

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