dt-bindings: add DT binding for the Aardvark PCIe controller

Add the documentation for the Device Tree binding for the Aardvark PCIe
controller, found on Marvell Armada 3700 ARM64 SoCs.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>