m68knommu: fix tick timer definition for coldfire

CLOCK_TICK_RATE should give the underlying frequency of the tick timer,
to make ntp happy.  For Coldfires, that's the main clock.

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>