dmaengine: Couple DMA channels to their physical DMA device

Set the 'parent' field of channel class devices to point to the
physical DMA device initialized by the DMA engine driver.

This allows drivers to use chan->dev.parent for syncing DMA buffers
and adds a 'device' symlink to the real device in
/sys/class/dma/dmaXchanY.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>