soc/rtc: add a function to wait for slow clock cycle

Some RTC features are synchronized to RTC_SLOW_CLK, so sometimes
software needs to wait for the next slow clock cycle.
This function implements waiting using Timer Group clock calibration
feature.

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

@@ -400,6 +400,15 @@ uint64_t rtc_time_slowclk_to_us(uint64_t rtc_cycles, uint32_t period);
  */
 uint64_t rtc_time_get();
 
+/**
+ * @brief Busy loop until next RTC_SLOW_CLK cycle
+ *
+ * This function returns not earlier than the next RTC_SLOW_CLK clock cycle.
+ * In some cases (e.g. when RTC_SLOW_CLK cycle is very close), it may return
+ * one RTC_SLOW_CLK cycle later.
+ */
+void rtc_clk_wait_for_slow_cycle();
+
 /**
  * @brief sleep configuration for rtc_sleep_init function
  */