m68knommu: improve the clock configuration defaults

Create some intelligent default settings for each ColdFire SoC type
in the configuration entry for CONFIG_CLOCK_FREQ.

The ColdFire clock frequency is configurable at build time. There is a
lot of variation in the frequency of operation on specific ColdFire based
boards. But we can choose a default that matches the maximum frequency
of clock operation for a particular ColdFire part. That is typically
the most common clock setting.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>