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
```diff
+    default:
+        return -1;
```
To get rid of cppcheck warning.