dmaengine: mpc512x: register for device tree channel lookup

Register the controller for device tree based lookup of DMA channels
(non-fatal for backwards compatibility with older device trees) and
provide the '#dma-cells' property in the shared mpc5121.dtsi file

Signed-off-by: Alexander Popov <a13xp0p0v88@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>