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
I was thinking about something like
```C
#define GPIO_NO_EXTI  (18)
```
and then use it as in
```C
#define GPIO_4_EXTINT      GPIO_NO_EXTI
```
With the current approach using `-1` I'm receiving warnings from the compiler complaining about negative left shifts. Using the approach above the code tries to enable an interrupt bit >17 in i.e. the `INTENSET` register which doesn't have any effect. Also you don't need the check `if (exti > GPIO_EXTINT_NUMF)`.