feat(esp_system): support XTAL as RTC fast clock source

components/esp_system/port/soc/esp32s31/clk.c

@@ -56,7 +56,16 @@ __attribute__((weak)) void esp_clk_init(void)
     assert(rtc_clk_xtal_freq_get() == SOC_XTAL_FREQ_40M);
 
     rtc_clk_8m_enable(true);
+#if CONFIG_RTC_FAST_CLK_SRC_RC_FAST
     rtc_clk_fast_src_set(SOC_RTC_FAST_CLK_SRC_RC_FAST);
+#elif CONFIG_RTC_FAST_CLK_SRC_XTAL
+    rtc_clk_fast_src_set(SOC_RTC_FAST_CLK_SRC_XTAL);
+    if (esp_sleep_sub_mode_dump_config(NULL)[ESP_SLEEP_RTC_FAST_USE_XTAL_MODE] == 0) {
+        esp_sleep_sub_mode_config(ESP_SLEEP_RTC_FAST_USE_XTAL_MODE, true);
+    }
+#else
+#error "No RTC fast clock source configured"
+#endif
 
 #if defined(CONFIG_BOOTLOADER_WDT_ENABLE) && SOC_RTC_WDT_SUPPORTED
     // WDT uses a SLOW_CLK clock source. After a function select_rtc_slow_clk a frequency of this source can changed.

components/ulp/test_apps/.build-test-rules.yml

@@ -4,9 +4,6 @@ components/ulp/test_apps/lp_core/lp_core_basic_tests:
   disable:
     - if: SOC_LP_CORE_SUPPORTED != 1
     - if: CONFIG_NAME == "xtal" and SOC_CLK_LP_FAST_SUPPORT_XTAL != 1
-    - if: IDF_TARGET in ["esp32s31"] and CONFIG_NAME == "xtal"
-      temporary: true
-      reason: CONFIG_RTC_FAST_CLK_SRC_XTAL not supported yet # TODO: [ESP32S31] IDF-15576
     - if: CONFIG_NAME == "lp_vad" and SOC_LP_VAD_SUPPORTED != 1
   depends_components:
     - ulp