pub fn xtal_freq() -> XtalClock {
        match Self::read_xtal_freq_mhz() {
            None | Some(32) => XtalClock::_32M,
            Some(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/esp32h2.rs