feat(freertos): add functions to better measure the CPU usage (#6619)

src/osal/lv_freertos.h

@@ -77,6 +77,29 @@ typedef struct {
  * GLOBAL PROTOTYPES
  **********************/
 
+/**
+ * Set it for `traceTASK_SWITCHED_IN()` as
+ * `lv_freertos_task_switch_in(pxCurrentTCB->pcTaskName)`
+ * to save the start time stamp of a task
+ * @param name      the name of the which is switched in
+ */
+void lv_freertos_task_switch_in(const char * name);
+
+/**
+ * Set it for `traceTASK_SWITCHED_OUT()` as
+ * `lv_freertos_task_switch_out()`
+ * to save finish time stamp of a task
+ */
+void lv_freertos_task_switch_out(void);
+
+/**
+ * Set it for `LV_SYSMON_GET_IDLE` to show the CPU usage
+ * as reported based the usage of FreeRTOS's idle task
+ * If it's important when a GPU is used.
+ * @return the idle percentage since the last call
+ */
+uint32_t lv_os_get_idle_percent(void);
+
 /**********************
  *      MACROS
  **********************/