powerpc/85xx: Remove DATARATE_*_MHZ defines in static ddr init

Rather than having #defines DATARATE_*_MHZ, lets just match what we do on
the SPD code and convert the DDR frequency into MHZ and just compare
with a constant.

Based on patch from Poonam Aggrwal.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>