ARM: Kirkwood: Add configuration for MPP12 as GPIO

The MPP12 is listed in the 6281 HW manual as output only but the iconnect
board from iomega is using it as GPIO (there's a button connected on it). So,
I'm adding a definition for the MPP12 as GPIO. As I've no informations about
this and which kirkwood are "affected", I'm adding a new #define instead of
modifying the current one for MPP12.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Nicolas Pitre <nico@fluxnic.net>