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
Even if you did not implement the SPI slave, could you also "implement" `spi_transmission_begin` (like `spi_init_slave`) because the SPI test includes this..