Merge branch 'feature/add_get_rtc_clk' into 'master'

feat(esp8266): Add get RTC clock value

See merge request sdk/ESP8266_RTOS_SDK!1507

components/esp8266/source/esp_sleep.c

@@ -427,3 +427,8 @@ esp_err_t esp_pm_configure(const void* vconfig)
     }
     return ESP_OK;
 }
+
+uint32_t rtc_time_get(void)
+{
+    return REG_READ(RTC_SLP_CNT_VAL);
+}