AVR32: Must add NOPs after disabling interrupts for AT32UC3A0512ES

The AT32UC3A0512ES chip has a bug when disabling interrupts. As a
workaround, two NOPs can be inserted.

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>