perf: added perf_event_count() method

this allows drivers to get an event_count from a perf counter

src/modules/systemlib/perf_counter.h

@@ -135,6 +135,14 @@ __EXPORT extern void		perf_print_all(void);
  */
 __EXPORT extern void		perf_reset_all(void);
 
+/**
+ * Return current event_count
+ *
+ * @param handle		The counter returned from perf_alloc.
+ * @return			event_count
+ */
+__EXPORT extern uint64_t	perf_event_count(perf_counter_t handle);
+
 __END_DECLS
 
 #endif