dma: mxs-dma: move to generic device tree binding

Update mxs-dma driver to adopt generic DMA device tree binding.  It
calls of_dma_controller_register() with mxs specific of_dma_xlate to
get the generic DMA device tree helper support.  Then DMA clients only
need to call dma_request_slave_channel() for requesting a DMA channel
from dmaengine.

The existing way of requesting channel, clients directly call
dma_request_channel(), still work there, and will be removed after
all mxs-dma clients get converted to generic DMA device tree helper.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>