rpiclock_get_frequency
uint32_t rpiclock_get_frequency(void *, uint32_t *);
sc->sc_cd.cd_get_frequency = rpiclock_get_frequency;
parent_freq = rpiclock_get_frequency(sc, &idx);
freq = rpiclock_get_frequency(sc, &parent);
best_freq = rpiclock_get_frequency(sc, cells);
parent_freq = rpiclock_get_frequency(sc, &parent);