ata: allow subsystem to be used on m68k arch

When libata was merged m68k lacked IOMAP support.  This has not been
true for a long time now so allow subsystem to be used on m68k.

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Tejun Heo <tj@kernel.org>