dmaengine: Add support for multiple descriptors for imx-dma.

dmaengine specifies the possibility that several descriptors
can be queued for transfer. It also indicates that tasklets
must be used for DMA callbacks.

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