dmaengine: documentation to the new callback mechanism

			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,