cpu/sam0_common: implement EXTWAKE for SAM L21
cpu/sam0_common: implement EXTWAKE for SAM L21

On SAM L21 only 8 EXTWAKE pins can wake the CPU from Backup sleep.
Handle this analogous to the RTC tamper pins on SAM D5x/E5x where
configuring them as an interrupt will also cause them to wake the
device from Deep Sleep.

miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,jasonatran/RIOT,ant9000/RIOT,kYc0o/RIOT,OlegHahm/RIOT,ant9000/RIOT,miri64/RIOT,OlegHahm/RIOT,miri64/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,jasonatran/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,OlegHahm/RIOT,miri64/RIOT,kYc0o/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,miri64/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT