ztimer_clock are meant to be chained. At the end of the chaine
there is always a timer device object (periph_rtt/rtc/timer).
Since ZTIMER_MSEC and ZTIMER_USEC can be the scaled/shifted with
respect to the base periph_rtt/rtc/timer it makes sense to chain
other ZTIMER_XXX on top of the base rtc/timer/rtt in order to avoid
chained convertions.