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.

include/libopencm3/efm32/ezr32wg/uart.h

@@ -1,3 +1,11 @@
+/** @defgroup uart_defines UART Defines
+ *
+ * @brief <b>Defined Constants and Types for the UART module</b>
+ *
+ * @ingroup EZR32WG_defines
+ *
+ * LGPL License Terms @ref lgpl_license
+ */
 /*
  * This file is part of the libopencm3 project.
  *
@@ -17,9 +25,6 @@
  * along with this library.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#ifndef LIBOPENCM3_EFM32_EZR32WG_UART_H
-#define LIBOPENCM3_EFM32_EZR32WG_UART_H
+#pragma once
 
 #include <libopencm3/efm32/common/uart_common.h>
-
-#endif