mpc8xxx: DDR2/DDR3: Clean up DIMM-type switch statements

mpc8xxx: DDR2/DDR3: Clean up DIMM-type switch statements

The numeric constants in the switch statements are replaced by #defines
added to the common ddr_spd.h header.  This dramatically improves the
readability of the switch statments.

In addition, a few of the longer lines were cleaned up, and the DDR2
type for an SO-RDIMM module was added to the DDR2 switch statement.

Signed-off-by: Kyle Moffett <[email protected]>
Cc: Andy Fleming <[email protected]>
Cc: Kim Phillips <[email protected]>
Acked-by: York Sun <[email protected]>
Signed-off-by: Kumar Gala <[email protected]>