mtd/maps: Fix accidental removal in Makefile

The commit d79c326c

 ("gpio-addr-flash: new driver for GPIO assisted
flash addressing") removed two lines from the Makefile by accident.
Though I'm not sure how this accident happened, this patch reverts the
removal.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>