esp_clk.h: add esp_clk_xtal_freq

This adds a user API for getting the XTAL frequency

components/esp32/include/esp_clk.h

@@ -62,6 +62,17 @@ int esp_clk_cpu_freq(void);
  */
 int esp_clk_apb_freq(void);
 
+/**
+ * @brief Return frequency of the main XTAL
+ *
+ * Frequency of the main XTAL can be either auto-detected or set at compile
+ * time (see CONFIG_ESP32_XTAL_FREQ_SEL sdkconfig option). In both cases, this
+ * function returns the actual value at run time.
+ *
+ * @return XTAL frequency, in Hz
+ */
+int esp_clk_xtal_freq(void);
+
 
 /**
  * @brief Read value of RTC counter, converting it to microseconds