powerpc/mpc8xxx: Add DDR2 to unified DDR driver

DDR2 has different ODT table and values. Adding table according to Samsung
application note.

Fix additive latency calculation to avoid interger underflow.

Also converted typedef dynamic_odt_t to struct dynamic_odt.

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