a3700_periph_sb_get_frequency
sc->sc_cd.cd_get_frequency = a3700_periph_sb_get_frequency;
freq = a3700_periph_sb_get_frequency(sc, &idx);
freq = a3700_periph_sb_get_frequency(sc, &idx);
uint32_t a3700_periph_sb_get_frequency(void *, uint32_t *);