m68k: move definition of THREAD_SIZE into thread_info_mm.h

Move the definition of THREAD_SIZE from page_mm.h to thread_info_mm.h
This logically associates it with the other thread definitions, and will
make it easier to merge the MMU and non-MMU versions of page.h and
thread_info.h.

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