dma: imx-sdma: Use struct for driver data

Use a struct type instead of an enum type for distinguishing between
different versions. This makes it simpler to handle multiple differences
without cluttering the code with comparisons for certain devtypes.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>