m68k: use ColdFire MMU read/write bit flags when ioremapping

The ColdFire MMU has separate read and write bits, unlike the Motorola
m68k MMU which has a single read-only bit.

Define a _PAGE_READWRITE value for the Motorola MMU, which is 0, so we
can unconditionaly include that in the page table entry bits when setting
up ioremapped pages.

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>