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 think you just need to use the `transceiver` module if the `netdev` is not available. You could do something like this:
```
ifneq (,$(filter defaulttransceiver,$(USEMODULE)))
    USEMODULE += at86rf231
    ifeq (,$(filter netdev_base,$(USEMODULE)))
        USEMODULE += transceiver
    endif
endif
```