PPC4xx: Add 440EPx DDR2 SPD DIMM support

This patch adds SPD DDR2 support for the 440EPx ("Denali") SDRAM controller. It should also work on the 440GRx. It is based on the DDR2 SPD code for the 440EP/440EPx, but makes no provision for DDR1 support.

This code has been tested on prototype Korat boards with three Kingston
DIMMS: 512 MiB ECC (one rank), 512 MiB non-ECC (one rank) and 1 GiB ECC (two ranks). The Korat board has a single DIMM socket, but support has been provided (though not tested) for boards with two DIMM sockets.

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[This patch creates a single file, but slightly exceeds the 40 KiB limit for the mailing list. It is available at "http://k1lj.home.att.net/u-boot/2007102400-patch3.txt".]