dt-bindings: PCI: Add PCI EP DT binding documentation for AM654

Add devicetree binding documentation for PCIe in EP mode present in
AM654 SoC.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>