dmaengine: Add support for MEMCPY for imx-dma.

MEMCPY transfers allow DMA copies from memory to
memory. This patch has been tested with dmatest
device driver.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>