cpu/samd21: add rtc periph implementation

Implementation of RTC for the SAMR21 board
It uses GCLK2 with OSC32K

@LudwigOrtmann, it should be clean now
Here, check and return an error.
      if (!(rtcMode2->INTENSET.reg & RTC_MODE2_INTENSET_ALARM0)){
		return -1;
	}
	while(rtcMode2->STATUS.bit.SYNCBUSY);