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
I think this is a good idea! Buf if `temp` is not used the pure read access `pi_dev->DATA.reg` should be enough. Not?