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
also the last three defines `PIN_RFCTRL1`, `PIN_RFCTRL2` and `RFCTRL_CFG_ANT_DIV` should be moved/removed?