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
Clear the interrupt flag of the given GPIO:
EIC->INTFLAG.reg = EIC_INTFLAG_EXTINTX;