pub fn rtc_calib_cal_mv(_unit: u8, atten: Attenuation) -> u16 {
        match atten {
            Attenuation::_0dB => 400,
            Attenuation::_2p5dB => 550,
            Attenuation::_6dB => 750,
            Attenuation::_11dB => 1370,
        }
    }
Get ADC reference point voltage for specified attenuation in millivolts

see <https://github.com/espressif/esp-idf/blob/903af13e8/components/efuse/esp32c6/esp_efuse_rtc_calib.c#L42>
esp-hal/src/soc/esp32c6/efuse/mod.rs