dt-bindings: PCI: cadence: Add DT bindings for Cadence PCIe endpoint controller

This patch documents the DT bindings for the Cadence PCIe controller
when configured in endpoint mode.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@free-electrons.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>