m68k/m68knommu: merge MMU and non-MMU atomic.h

The only difference between the MMU and non-MMU versions of atomic.h
is some extra support needed by ColdFire family processors. So merge
this into the MMU version of atomic.h.

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