cpu/sam0_common/rtt: add configurable RTT_FREQUENCY
cpu/sam0_common/rtt: add configurable RTT_FREQUENCY

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