dt-bindings: PCI: designware: Add binding for Designware PCIe in ECAM mode

Describe the binding for firmware-configured instances of the Synopsys
DesignWare PCIe controller in RC mode, that are almost but not quite ECAM
compliant.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>