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
maybe introduce `#define GPIO_NO_EXTI (18)`, this way there are no warnings when compiling and setting a bit above 17 in `EIC->INT*` has no effect.