samr21: implemention of transceiver via spi

Implementaion of the at86rf231 driver for the samr21 chip this include
 - A gpio fix
 - An SPI implementation with SPI_0 being used for the internal transceiver. Does not support more than 1Mhz transfer
If it should be possible to use the GPIOs > 3 just as GPIOs and independent from SPI, it could be clearer to define them like the others and don't refert to the SPI values.