doc:stm32: usart: fix grouping and heirarchy of base addrs

They were always landing on the top level, or not even present.

include/libopencm3/stm32/l0/usart.h

@@ -30,8 +30,9 @@
 #include <libopencm3/stm32/common/usart_common_all.h>
 #include <libopencm3/stm32/common/usart_common_v2.h>
 
+/**@{*/
+
 /** @defgroup usart_reg_base USART register base addresses
- * @ingroup STM32F_usart_defines
  * Holds all the U(S)ART peripherals supported.
  * @{
  */
@@ -46,4 +47,5 @@ BEGIN_DECLS
 
 END_DECLS
 
+/**@}*/
 #endif