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
Please add something like proposed by @haukepetersen until we have a proper clk source so the SPI clk can be more than 1MHz