powerpc/p2041rdb: add more ddr frequencies support

This table covers DDR frequencies from 666 to 1666. Frequencies 666, 833,
1000, 1066 and 1333 were verified on this board with SO-DIMM
(UG51U6400N8SU-ACF).

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>