ppc4xx: Disable DDR2 autocalibration on Kilauea for now

Since the new autocalibration still has some problems on some Kilauea
boards with 200MHz DDR2 frequency we disable the autocalibration and
use the hardcoded values as done before. This seems to work reliably
on all known DDR2 frequencies.

After the autocalibration issue is fixed we will enable it again.

Signed-off-by: Stefan Roese <sr@denx.de>