dmaengine: xilinx: dpdma: add missing kernel doc

xilinx_dpdma_sw_desc_set_dma_addrs() documentation is missing describing
'xdev', so add it

drivers/dma/xilinx/xilinx_dpdma.c:313: warning: Function parameter or
member 'xdev' not described in 'xilinx_dpdma_sw_desc_set_dma_addrs'

Link: https://lore.kernel.org/r/20200718135201.191881-2-vkoul@kernel.org


Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>