dt-bindings: PCI: Add Qualcomm PCIe Endpoint controller

Add devicetree binding for Qualcomm PCIe Endpoint controller used in
platforms like SDX55. The Endpoint controller is based on the DesignWare
core with Qualcomm-specific wrappers.

Link: https://lore.kernel.org/r/20210920065946.15090-2-manivannan.sadhasivam@linaro.org
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>