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

This commit adds the Device Tree binding documentation for the Marvell
XOR v2 engine, which is found on Marvell Armada 7K/8K ARM64 SoCs.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>