ARM: mxc: clean up imx-dma device registration

The patch follows the implementation of gpio-mxc device registration
to break the concentrated imx-dma device registration into soc
specific setup function.  Then we can avoid the churn of "#ifdef"
and the cpu_is_mx checking on such a long list, which makes no sense,
considering more soc supports need to be added and we need to support
single image for multiple socs in the long run.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>