dt-bindings: dma: add binding for the Marvell XOR v2 engine

* Marvell XOR v2 engines

Required properties:

- compatible: one of the following values:

    "marvell,armada-7k-xor"

    "marvell,xor-v2"

- reg: Should contain registers location and length (two sets)

    the first set is the DMA registers

    the second set is the global registers

- msi-parent: Phandle to the MSI-capable interrupt controller used for

  interrupts.

Optional properties:

- clocks: Optional reference to the clock used by the XOR engine.

Example:

	xor0@400000 {

		compatible = "marvell,xor-v2";

		reg = <0x400000 0x1000>,

		      <0x410000 0x1000>;

		msi-parent = <&gic_v2m0>;

		dma-coherent;

	};