stm32: usart: fix doxygen for h7 at least

The usart_common_fifos uses a very nice style of docs in the headers, so
inline help works in some editors, without having to have the source of
the library available as well.  However, it means that the group
definition with the name doesn't appear until later, and then the title
is ignored.  Move the description to the header definition instead.

include/libopencm3/stm32/common/usart_common_fifos.h

@@ -92,8 +92,10 @@ typedef enum {
 
 BEGIN_DECLS
 
-/** @addtogroup usart_file
-@{*/
+/** @defgroup usart_file USART peripheral API
+ * @ingroup peripheral_apis
+ * @{
+ */
 /**
  * Enable FIFOs on the specified USART.
  * @param[in] usart  Base address of USART to configure FIFOs.