cpu/samd21: add rtc periph implementation

Implementation of RTC for the SAMR21 board
It uses GCLK2 with OSC32K
To explain:
This is a problem in the GitHub interface - it shows diffs against the master of the fork (bapclenet/RIOT.git/master in this case), not against the branch it is going to be merged into (in this case RIOT-OS/RIOT.git/master).