dmaengine: dmatest: Add support for completion polling

dmaengine: dmatest: Add support for completion polling

With the polled parameter the DMA drivers can be tested if they can work
correctly when no completion is requested (no DMA_PREP_INTERRUPT and no
callback is provided).

If polled mode is selected then use dma_sync_wait() to execute the test
iteration instead of relying on the completion callback.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20190731071438.24075-1-peter.ujfalusi@ti.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>