dmaengine: zynqmp_dma: Fix kernel doc-format

This patch fixes the below kernel doc warnings
drivers/dma/xilinx/zynqmp_dma.c:552: info: Scanning doc for
zynqmp_dma_device_config
drivers/dma/xilinx/zynqmp_dma.c:558: warning: No description found for
return value of 'zynqmp_dma_device_config'
drivers/dma/xilinx/zynqmp_dma.c:649: info: Scanning doc for
zynqmp_dma_free_descriptors
drivers/dma/xilinx/zynqmp_dma.c:653: warning: No description found for
parameter 'chan'
drivers/dma/xilinx/zynqmp_dma.c:653: warning: Excess function parameter
'dchan' description in 'zynqmp_dma_free_descriptors'

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>