m68knommu: reorganize the no-MMU cache flushing to match m68k

Introduce cache_push() and cache_clear() functions for the non-MMU m68k
devices. With these in place we can more easily merge some of the common
m68k arch code.

In particular by reorganizing the __flush_cache_all() code and separating
the cache push and clear functions it becomes trivial to implement the
new cache_push() and cache_clear() functions.

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