dt-bindings: pci: add DT binding for Marvell Armada 7K/8K PCIe controller

This commit adds the Device Tree binding documentation that allows to
describe the PCIe controller found in Marvell Armada 7K/8K SoCs.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Rob Herring <rob@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>