doc: efm32: uart/usart provide common doxygen

Fix some direct includes, tag properly, include a stub file to pull in
shared header documentation and re-arrange some existing documentation
to make it present nicely.

lib/efm32/common/usart_common.c

@@ -0,0 +1,18 @@
+/** @addtogroup usart_file UART/USART peripheral API
+ * @ingroup peripheral_apis
+ * @brief UART/USART helper functions.
+ *
+ * <b>NO</b> helper functions exist. Only header definitions are available.
+ * Delete these lines if/when you add actual helper APIs.
+ * @sa usart_defines
+ * @sa uart_defines
+ * @copyright See @ref lgpl_license
+ */
+
+#include <libopencm3/efm32/uart.h>
+#include <libopencm3/efm32/usart.h>
+
+/**@{*/
+
+/**@}*/
+