dmaengine: mpc512x: add device tree binding document

* 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.