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
when we start using modulo operators here, it seems to me that the pin configuration in the `periph_conf.h` is off. I guess it would be smarter to define the pin as numeric value (e.g. 13, 7..) in the `periph_conf` and then use something like `(1 << pin)` in case a bitmask is needed. This eliminates all these (very expensive) modulo operations...