powerpc/mpc8xxx: Merge entries in DDR speed table

It is not necessary to keep multiple entries for the same setting in DDR
speed tables. Merge them for smaller tables. Also restructure the tables
for smaller size. Cleanup some typedefs.

Enforce strict checking for speed table. If DIMM is running at higher than
known speed, try to use the highest speed setting. If rank is unknown, it
has to panic.

Removed ODT overriding for P2020DS as it is not necessary.

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>