dmaengine: Add helper function to convert direction value to text

dmaengine_get_direction_text() can be useful when the direction is printed
out. The text is easier to comprehend than the number.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20191223110458.30766-7-peter.ujfalusi@ti.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>