Arduino due spi drivers

This PR provides SPI drivers for the Arduino-Due board and a project that tests these drivers. 
You need to guard the file with 
```c
#if SPI_NUMOF
```
and the corresponding `#endif` in the end of the file. Otherwise boards that use this CPU would try to compile this file even if they don't have an SPI device defined which will then end up in missing types etc.

This should also take care of the failed Travis tests for the Udoo board.