m68knommu: factor more common ColdFire cpu reset code

Most of the more modern ColdFire cores use the same code to reset the CPU
(but it is different to most of the earlier cores). Currently that is
duplicated in each of the sub-arch files. Pull out this common code and
out a single copy of it with the other common reset code.

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