Add support for 32k XTAL as RTC_SLOW_CLK source

- RTC_CNTL_SLOWCLK_FREQ define is removed; rtc_clk_slow_freq_get_hz
  function can be used instead to get an approximate RTC_SLOW_CLK
  frequency

- Clock calibration is performed at startup. The value is saved and used
  for timekeeping and when entering deep sleep.

- When using the 32k XTAL, startup code will wait for the oscillator to
  start up. This can be possibly optimized by starting a separate task
  to wait for oscillator startup, and performing clock switch in that
  task.

- Fix a bug that 32k XTAL would be disabled in rtc_clk_init.

- Fix a rounding error in rtc_clk_cal, which caused systematic frequency
  error.

- Fix an overflow bug which caused rtc_clk_cal to timeout early if the
  slow_clk_cycles argument would exceed certain value

- Improve 32k XTAL oscillator startup time by introducing bootstrapping
  code, which uses internal pullup/pulldown resistors on 32K_N/32K_P
  pins to set better initial conditions for the oscillator.

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

@@ -168,6 +168,15 @@ void rtc_clk_32k_enable(bool en);
  */
 bool rtc_clk_32k_enabled();
 
+/**
+ * @brief Enable 32k oscillator, configuring it for fast startup time.
+ * Note: to achieve higher frequency stability, rtc_clk_32k_enable function
+ * must be called one the 32k XTAL oscillator has started up. This function
+ * will initially disable the 32k XTAL oscillator, so it should not be called
+ * when the system is using 32k XTAL as RTC_SLOW_CLK.
+ */
+void rtc_clk_32k_bootstrap();
+
 /**
  * @brief Enable or disable 8 MHz internal oscillator
  *
@@ -229,6 +238,20 @@ void rtc_clk_slow_freq_set(rtc_slow_freq_t slow_freq);
  */
 rtc_slow_freq_t rtc_clk_slow_freq_get();
 
+/**
+ * @brief Get the approximate frequency of RTC_SLOW_CLK, in Hz
+ *
+ * - if RTC_SLOW_FREQ_RTC is selected, returns ~150000
+ * - if RTC_SLOW_FREQ_32K_XTAL is selected, returns 32768
+ * - if RTC_SLOW_FREQ_8MD256 is selected, returns ~33000
+ *
+ * rtc_clk_cal function can be used to get more precise value by comparing
+ * RTC_SLOW_CLK frequency to the frequency of main XTAL.
+ *
+ * @return RTC_SLOW_CLK frequency, in Hz
+ */
+uint32_t rtc_clk_slow_freq_get_hz();
+
 /**
  * @brief Select source for RTC_FAST_CLK
  * @param fast_freq clock source (one of rtc_fast_freq_t values)