doc: efm32: emu: tag for doxygen properly

Requires a stub .c file to make the common files be included until we
build some APIs.

lib/efm32/common/emu_common.c

@@ -0,0 +1,15 @@
+/** @addtogroup emu_file EMU peripheral API
+ * @ingroup peripheral_apis
+ * @brief Energy Management Unit 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/emu.h>
+
+/**@{*/
+
+/**@}*/
+