AVR32: Make GPIO implmentation cpu dependent

There are some differences in the implementation of GPIO in the
at32uc chip compared to the ap700x series.

Signed-off-by: Gunnar Rangoy <gunnar@rangoy.com>
Signed-off-by: Paul Driveklepp <pauldriveklepp@gmail.com>
Signed-off-by: Olav Morken <olavmrk@gmail.com>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>