docs(gptimer): include etm content conditionally

https://www.esp32.com/viewtopic.php?f=12&t=40991&p=135107#p135098

components/esp_driver_gptimer/include/driver/gptimer.h

@@ -117,7 +117,8 @@ esp_err_t gptimer_get_resolution(gptimer_handle_t timer, uint32_t *out_resolutio
 /**
  * @brief Get GPTimer captured count value
  *
- * @note The capture action can be issued either by ETM event or by software (see also `gptimer_get_raw_count`).
+ * @note Different from `gptimer_get_raw_count`, this function won't trigger a software capture event. It just returns the last captured count value.
+ *       It's especially useful when the capture has already been triggered by an external event and you want to read the captured value.
  * @note This function is allowed to run within ISR context
  * @note If `CONFIG_GPTIMER_CTRL_FUNC_IN_IRAM` is enabled, this function will be placed in the IRAM by linker,
  *       makes it possible to execute even when the Flash Cache is disabled.

components/esp_driver_gptimer/include/driver/gptimer_etm.h

@@ -9,11 +9,14 @@
 #include "esp_err.h"
 #include "esp_etm.h"
 #include "driver/gptimer_types.h"
+#include "soc/soc_caps.h"
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+#if SOC_TIMER_SUPPORT_ETM
+
 /**
  * @brief GPTimer ETM event configuration
  */
@@ -58,6 +61,7 @@ typedef struct {
  */
 esp_err_t gptimer_new_etm_task(gptimer_handle_t timer, const gptimer_etm_task_config_t *config, esp_etm_task_handle_t *out_task);
 
+#endif // SOC_TIMER_SUPPORT_ETM
 #ifdef __cplusplus
 }
 #endif