dmaengine: enable mxs-dma for imx6ul

The mxs-dma unit is also available on i.MX6UL. Make it possible to
select it in Kconfig.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>