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

The MMU and non-MMU versions of thread_info.h are quite similar.
Merge the two files.

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