dmaengine: xilinx: dpdma: Fix kerneldoc warning

Document the struct xilinx_dpdma_chan desc field to fix a kerneldoc
undocumented member warning (which can be reproduced by compiling with
W=1).

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20200722161747.30048-1-laurent.pinchart@ideasonboard.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/xilinx/xilinx_dpdma.c

@@ -214,6 +214,7 @@ struct xilinx_dpdma_tx_desc {
  * @lock: lock to access struct xilinx_dpdma_chan
  * @desc_pool: descriptor allocation pool
  * @err_task: error IRQ bottom half handler
+ * @desc: References to descriptors being processed
  * @desc.pending: Descriptor schedule to the hardware, pending execution
  * @desc.active: Descriptor being executed by the hardware
  * @xdev: DPDMA device