dmaengine: mpc512x: add device tree binding document

Introduce a device tree binding document for the MPC512x DMA controller
Signed-off-by: Alexander Popov <a13xp0p0v88@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

Documentation/devicetree/bindings/dma/mpc512x-dma.txt

+* Freescale MPC512x and MPC8308 DMA Controller
+
+The DMA controller in Freescale MPC512x and MPC8308 SoCs can move
+blocks of memory contents between memory and peripherals or
+from memory to memory.
+
+Refer to "Generic DMA Controller and DMA request bindings" in
+the dma/dma.txt file for a more detailed description of binding.
+
+Required properties:
+- compatible: should be "fsl,mpc5121-dma" or "fsl,mpc8308-dma";
+- reg: should contain the DMA controller registers location and length;
+- interrupt for the DMA controller: syntax of interrupt client node
+	is described in interrupt-controller/interrupts.txt file.
+- #dma-cells: the length of the DMA specifier, must be <1>.
+	Each channel of this DMA controller has a peripheral request line,
+	the assignment is fixed in hardware. This one cell
+	in dmas property of a client device represents the channel number.
+
+Example:
+
+	dma0: dma@14000 {
+		compatible = "fsl,mpc5121-dma";
+		reg = <0x14000 0x1800>;
+		interrupts = <65 0x8>;
+		#dma-cells = <1>;
+	};
+
+DMA clients must use the format described in dma/dma.txt file.