doc: efm32: msc: tag for doxygen inclusion

Includes a stub .c file to trigger common code generation

lib/efm32/common/msc_common.c

@@ -0,0 +1,15 @@
+/** @addtogroup msc_file MSC peripheral API
+ * @ingroup peripheral_apis
+ * @brief Memory Systems Controller 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/msc.h>
+
+/**@{*/
+
+/**@}*/
+