m68knommu: fix clock rate value reported for ColdFire 54xx parts

The instruction timings of the ColdFire 54xx family parts are
different to other version 4 parts (or version 2 or 3 parts for
that matter too).

Move the instruction timing setting into the ColdFire part
specific headers, and set the 54xx value appropriately.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>