drivers/periph/rtt.h: add RTT_FREQUENCY & RTT_MAX_VALUE doc
drivers/periph/rtt.h: add RTT_FREQUENCY & RTT_MAX_VALUE doc

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