soc: implement XTAL frequency detection

ROM code already implements XTAL frequency detection, but it uses the 8M
clock before the clock tuning parameters are initialized. With the
zero clock tuning parameter, 8M clock has significant frequency deviation
at high temperatures, which can lead to erroneous detection of 40 MHz
crystal as a 26 MHz one.

This change adds XTAL frequency detection code to rtc_clk_init routine,
and detection is performed after the 8M clock tuning parameter as been
initialized.

components/soc/esp32/include/soc/rtc.h

@@ -92,6 +92,9 @@ typedef enum {
     RTC_FAST_FREQ_8M = 1,       //!< Internal 8 MHz RC oscillator
 } rtc_fast_freq_t;
 
+/* With the default value of CK8M_DFREQ, 8M clock frequency is 8.5 MHz +/- 7% */
+#define RTC_FAST_CLK_FREQ_APPROX 8500000
+
 /**
  * @brief Clock source to be calibrated using rtc_clk_cal function
  */
@@ -307,6 +310,15 @@ uint32_t rtc_clk_apb_freq_get();
  */
 uint32_t rtc_clk_cal(rtc_cal_sel_t cal_clk, uint32_t slow_clk_cycles);
 
+/**
+ * @brief Measure ratio between XTAL frequency and RTC slow clock frequency
+ * @param cal_clk slow clock to be measured
+ * @param slow_clk_cycles number of slow clock cycles to average
+ * @return average ratio between XTAL frequency and slow clock frequency,
+ *         Q13.19 fixed point format, or 0 if calibration has timed out.
+ */
+uint32_t rtc_clk_cal_ratio(rtc_cal_sel_t cal_clk, uint32_t slow_clk_cycles);
+
 /**
  * @brief Convert time interval from microseconds to RTC_SLOW_CLK cycles
  * @param time_in_us Time interval in microseconds