m68knommu: make cache push code ColdFire generic

Currently the code to push cache lines is only available to version 4
cores. Version 3 cores may also need to use this if we support copy-
back caches on them. Move this code to make it more generic, and
useful for all version ColdFire cores.

With this in place we can now have a single cache_flush_all() code
path that does all the right things on all version cores.

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