cpu: Initial import of the arduino-due

This PR consists of three parts: 
- board: arduino-due
- cpu: cortex-m common
- cpu: sam3x8e

The port is not quite complete, but timers, uart and gpio are working.

For testing I think we only have one arduino-due board available so far - who is willing to test this can borrow it from my desk...

This PR is dependent on:
- #507
- #920
- #922
- #931