cpu/board: updated  spi driver for arduino-due

This PR should replace PR #1544 from @blanloem because his internship ended. It contains an updated and adapted version of the SPI low-level driver for arduino-due boards.
I don't think casting is the right approach here.
I mean - if length really exceeds int, the result is off now, and this is lost.
IFF this should be handled at all, I'd `length = (length > INT_MAX) INT_MAX : length;` (if INT_MAX is the right macro, this is off the top of my head).