cpu/sam0_common: PWM: fix doxygen group
cpu/sam0_common: PWM: fix doxygen group

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