boards/samd21-xpro: update PWM configuration
boards/samd21-xpro: update PWM configuration

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