ztimer: periph_rtt: ensure RTT_MAX_VALUE is honored
ztimer: periph_rtt: ensure RTT_MAX_VALUE is honored

Previously, ztimer would happily set an absolute RTT alarm value that exceeds
RTT's maximum value (though not a longer interval), as the `val` was
simply added to `rtt_get_counter()`.
This commit ensures that the target value wraps around RTT_MAX_VALUE.

Fixes #13920.

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