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/esp32/Kconfig

@@ -542,6 +542,23 @@ config ESP32_RTC_CLOCK_SOURCE_EXTERNAL_CRYSTAL
     bool "External 32kHz crystal"
 endchoice
 
+config ESP32_RTC_CLK_CAL_CYCLES
+    int "Number of cycles for RTC_SLOW_CLK calibration"
+    default 1024
+    range 0 125000
+    help
+        When the startup code initializes RTC_SLOW_CLK, it can perform
+        calibration by comparing the RTC_SLOW_CLK frequency with main XTAL
+        frequency. This option sets the number of RTC_SLOW_CLK cycles measured
+        by the calibration routine. Higher numbers increase calibration
+        precision, which may be important for applications which spend a lot of
+        time in deep sleep. Lower numbers reduce startup time.
+        
+        When this option is set to 0, clock calibration will not be performed at
+        startup, and approximate clock frequencies will be assumed:
+        - 150000 Hz if internal RC oscillator is used as clock source
+        - 32768 Hz if the 32k crystal oscillator is used
+
 config ESP32_DEEP_SLEEP_WAKEUP_DELAY
     int "Extra delay in deep sleep wake stub (in us)"
     default 0