doc: efm32: acmp: add missing file

Classssssssic error. Don't git add the local file created. doh.

lib/efm32/common/acmp_common.c

@@ -0,0 +1,15 @@
+/** @addtogroup acmp_file ACMP peripheral API
+ * @ingroup peripheral_apis
+ * @brief Analog Comparator 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/acmp.h>
+
+/**@{*/
+
+/**@}*/
+