ARM: mxs: do not add dma device by default

This will cause conflict when dt is enabled.
So let each platform add dma devices respectively.

Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Huang Shijie <b32955@freescale.com>
Signed-off-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Vinod Koul <vinod.koul@intel.com>