m68k: compile appropriate mm arch files for ColdFire MMU support

Create a config symbol to enable when using a ColdFire MMU. We then
use that to only compile the necessary arch mm files.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Matt Waddel <mwaddel@yahoo.com>
Acked-by: Kurt Mahan <kmahan@xmission.com>