stm32: dma: doxygen fixups

Use a single @defgroup for the "root" of a common heirarchy, and only
addtogroup for additions.  This prevents an alphabetically "first" entry
from being used as the documentation for the entire group.

lib/stm32/common/dma_common_csel.c

@@ -1,7 +1,4 @@
-/** @addtogroup dma_file DMA peripheral API
+/** @addtogroup dma_file
-@ingroup peripheral_apis
-
-LGPL License Terms @ref lgpl_license
  */
 /*
  * This file is part of the libopencm3 project.

lib/stm32/common/dma_common_f24.c

@@ -1,5 +1,6 @@
-/** @addtogroup dma_file DMA peripheral API
+/** @defgroup dma_file DMA peripheral API
 @ingroup peripheral_apis
+@brief DMA library for the multi stream controller found in f2/f4/f7 parts.
 
 @author @htmlonly © @endhtmlonly 2012
 Ken Sarkies <ksarkies@internode.on.net>

lib/stm32/common/dma_common_l1f013.c

@@ -1,5 +1,6 @@
-/** @addtogroup dma_file DMA peripheral API
+/** @defgroup dma_file DMA peripheral API
 @ingroup peripheral_apis
+@brief DMA library for the multi channel controller found in F0/1/3 & L/G parts.
 
 @author @htmlonly &copy; @endhtmlonly 2010 Thomas Otto <tommi@viadmin.org>