powerpc/85xx: Update fixed DDR3 timing table for P4080DS

Most of time U-boot doesn't get an exact clock number. For example, clock
900MHz may be detected as 899.99MHz. 800MHz could be 799.99MHz. Update the
table to align the desired clocks in the middle.

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