doc: efm32: letimer: include tags for doxygen

Include a stub .c file to document the shared headers.

lib/efm32/common/letimer_common.c

@@ -0,0 +1,15 @@
+/** @addtogroup letimer_file LETIMER peripheral API
+ * @ingroup peripheral_apis
+ * @brief Low Energy Timer helper functions.
+ *
+ * <b>NO</b> helper functions exist. Only header definitions are available.
+ * Delete these lines if/when you add actual helper APIs.
+ * @copyright See @ref lgpl_license
+ */
+
+#include <libopencm3/efm32/letimer.h>
+
+/**@{*/
+
+/**@}*/
+