dmaengine: documentation to the new callback mechanism

Adding documentation in dmaengine/provider.txt on the usage of callback
with result, callback_result, function pointer to retrieve transanction
result from a DMA submission.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

Documentation/dmaengine/provider.txt

@@ -282,6 +282,17 @@ supported.
 			that is supposed to push the current
 			transaction descriptor to a pending queue, waiting
 			for issue_pending to be called.
+     - In this structure the function pointer callback_result can be
+       initialized in order for the submitter to be notified that a
+       transaction has completed. In the earlier code the function pointer
+       callback has been used. However it does not provide any status to the
+       transaction and will be deprecated. The result structure defined as
+       dmaengine_result that is passed in to callback_result has two fields:
+       + result: This provides the transfer result defined by
+		 dmaengine_tx_result. Either success or some error
+		 condition.
+       + residue: Provides the residue bytes of the transfer for those that
+		  support residue.
 
    * device_issue_pending
      - Takes the first transaction descriptor in the pending queue,