dma: Move struct member doc comments to fields

To better cover the struct fields of the DMA API in doxygen the fields
are now individually documented rather than documenting them ad-hoc in
the struct header doc comment.

A best attempt at marking fields that are HW specific has been done as
well. That means almost all of them.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>