Documentation: dmaengine: Add DMA_CTRL_REUSE documentation

    - This can be acked by invoking async_tx_ack()

    - If set, does not mean descriptor can be reused

  * DMA_CTRL_REUSE

    - If set, the descriptor can be reused after being completed. It should

      not be freed by provider if this flag is set.

    - The descriptor should be prepared for reuse by invoking

      dmaengine_desc_set_reuse() which will set DMA_CTRL_REUSE.

    - dmaengine_desc_set_reuse() will succeed only when channel support

      reusable descriptor as exhibited by capablities

    - As a consequence, if a device driver wants to skip the dma_map_sg() and

      dma_unmap_sg() in between 2 transfers, because the DMA'd data wasn't used,

      it can resubmit the transfer right after its completion.

    - Descriptor can be freed in few ways

	- Clearing DMA_CTRL_REUSE by invoking dmaengine_desc_clear_reuse()

	  and submitting for last txn

	- Explicitly invoking dmaengine_desc_free(), this can succeed only

	  when DMA_CTRL_REUSE is already set

	- Terminating the channel

General Design Notes

--------------------