cpu/samd21: add rtc periph implementation

Implementation of RTC for the SAMR21 board
It uses GCLK2 with OSC32K

@LudwigOrtmann, it should be clean now
Shouldn't this be `(timer->tm_sec - 1)`?
Reference Manual 17.8.26
> ... When a match occurs, the Alarm 0 interrupt flag in the Interrupt Flag Status and Clear register (INTFLAG.ALARMn) is set on the **next** counter cycle, ...