clk_tree: added default clock source for peripheral

examples/peripherals/timer_group/gptimer/main/gptimer_example_main.c

@@ -75,7 +75,7 @@ void app_main(void)
     ESP_LOGI(TAG, "Create timer handle");
     gptimer_handle_t gptimer = NULL;
     gptimer_config_t timer_config = {
-        .clk_src = GPTIMER_CLK_SRC_APB,
+        .clk_src = GPTIMER_CLK_SRC_DEFAULT,
         .direction = GPTIMER_COUNT_UP,
         .resolution_hz = 1000000, // 1MHz, 1 tick=1us
     };