pub fn xtal_freq() -> XtalClock {
        match Self::xtal_freq_mhz() {
            40 => XtalClock::_40M,
            other => XtalClock::Other(other),
        }
    }
Get main XTAL frequency
This is the value stored in RTC register RTC_XTAL_FREQ_REG by the
bootloader, as passed to rtc_clk_init function.
esp-hal/src/rtc_cntl/rtc/esp32c6.rs