doc: efm32: timer: enable peripheral_apis style

lib/efm32/common/timer_common.c

@@ -1,3 +1,6 @@
+/** @addtogroup timer_file TIMER peripheral API
+ * @ingroup peripheral_apis
+ */
 /*
  * This file is part of the libopencm3 project.
  *
@@ -19,6 +22,8 @@
 
 #include <libopencm3/efm32/timer.h>
 
+/**@{*/
+
 #define HAS_DEAD_TIME_INSERTION(timer) (timer == TIMER0)
 
 /**
@@ -60,3 +65,5 @@ void timer_set_top(uint32_t timer, uint32_t top)
 {
 	TIMER_TOP(timer) = top;
 }
+
+/**@}*/