esp32: make time monotonic across resets

Small changes to clock calibration value will cause increasing errors
the longer the device runs. Consider the case of deep sleep, assuming
that RTC counter is used for timekeeping:
- before sleep:
   time_before = rtc_counter * calibration_val
- after sleep:
   time_after = (rtc_counter + sleep_count) * (calibration_val + epsilon)
where 'epsilon' is a small estimation error of 'calibration_val'.
The apparent sleep duration thus will be:
time_after - time_before = sleep_count * (calibration_val + epsilon)
                           + rtc_counter * epsilon

Second term on the right hand side is the error in time difference
estimation, it is proportional to the total system runtime (rtc_counter).

To avoid this issue, this change makes RTC_SLOW_CLK calibration value
persistent across restarts. This allows the calibration value update to
be preformed, while keeping time after update same as before the update.

components/esp32/include/rom/rtc.h

@@ -50,9 +50,9 @@ extern "C" {
   *     0x3ff80000(0x400c0000)  Fast    8192            deep sleep entry code
   *
   *************************************************************************************
-  *     Rtc store registers     usage
-  *     RTC_CNTL_STORE0_REG
-  *     RTC_CNTL_STORE1_REG
+  *     RTC store registers     usage
+  *     RTC_CNTL_STORE0_REG     Reserved
+  *     RTC_CNTL_STORE1_REG     RTC_SLOW_CLK calibration value
   *     RTC_CNTL_STORE2_REG     Boot time, low word
   *     RTC_CNTL_STORE3_REG     Boot time, high word
   *     RTC_CNTL_STORE4_REG     External XTAL frequency
@@ -62,6 +62,7 @@ extern "C" {
   *************************************************************************************
   */
 
+#define RTC_SLOW_CLK_CAL_REG    RTC_CNTL_STORE1_REG
 #define RTC_BOOT_TIME_LOW_REG   RTC_CNTL_STORE2_REG
 #define RTC_BOOT_TIME_HIGH_REG  RTC_CNTL_STORE3_REG
 #define RTC_XTAL_FREQ_REG       RTC_CNTL_STORE4_REG