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
it's used in transceiver.c for creating a buffer of ieee802154_packet_t. I looked at some of the other cpu's and they define it in cpu-conf as well.