m68k: define PAGE_OFFSET_RAW for ColdFire CPU with MMU enabled

The ColdFire CPU configurations need PAGE_OFFSET_RAW set to the base of
their RAM. It doesn't matter if they are running with the MMU enabled or
disabled, it is always set to the base of RAM.

We can keep the choices simple here and key of CONFIG_RAMBASE. If it is
defined we are on a plaftorm (ColdFire or other non-MMU systems) which
have a configurable RAM base, just use it.

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