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
This seems dangerous, as it touches other application already using this driver... I guess the best solution for this would be to make the SPI bus-speed configurable in the driver, maybe include something like on top of this file:
```
#ifndef AT86RF231_SPI_SPEED
#define SPI_SPEED     SPI_SPEED_5MHZ
#else
#define SPI_SPEED    AT86RF231_SPI_SPEED
#endif

spi_init_master(AT86RF231_SPI, SPI_CONF_FIRST_RISING, SPI_SPEED);
```
and then define the 1MHz speed value in the board.h together with the SPI port to use?!
@thomaseichinger : Could you look into this?