dt-bindings: PCI: Add DT bindings for PCI designware EP mode

Add device tree binding documentation for PCI designware EP mode.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>