dmaengine: owl: Fix warnings generated during build

Following warnings are generated when compiled with W=1,

drivers/dma/owl-dma.c:170: warning: Function parameter or member 'cyclic'
not described in 'owl_dma_txd'
drivers/dma/owl-dma.c:198: warning: Function parameter or member 'cfg' not
described in 'owl_dma_vchan'
drivers/dma/owl-dma.c:198: warning: Function parameter or member 'drq' not
described in 'owl_dma_vchan'
drivers/dma/owl-dma.c:225: warning: Function parameter or member 'irq' not
described in 'owl_dma'

Fix this by adding comments for relevant struct members to appear in
kernel-doc.

Fixes: d64e1b3f ("dmaengine: owl: Add Slave and Cyclic mode support for
Actions Semi Owl S900 SoC")

Reported-by: Vinod Koul <vinod.koul@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Vinod Koul <vkoul@kernel.org>