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
remove defines for `CS`, `IRQ0`, `RESET` and `SLEEP`. These don't have anything to do with the SPI driver, but only with the radio... You should configure these as `GPIO_x` devices and control them using the low-level GPIO driver.