stm32: doc: move timer to peripheral_apis

Drop all the dummy timer.c files, and _actually_ compile the L1 specific
file that was dropped by accident in 5b8953124e in 2013!

lib/stm32/l1/timer.c

@@ -1,7 +1,5 @@
-/** @defgroup timer_file Timers
-
-@ingroup STM32L1xx
-
+/** @defgroup timer_file
+@ingroup peripheral_apis
 @brief <b>libopencm3 STM32L1xx Timers</b>
 
 @version 1.0.0
@@ -30,10 +28,10 @@
  * along with this library.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-/**@{*/
-
 #include <libopencm3/stm32/timer.h>
 
+/**@{*/
+
 /*---------------------------------------------------------------------------*/
 /** @brief Set Timer Option