dt-bindings: PCI: cdns-ep: Fix 'unevaluatedProperties' warnings

dt-bindings: PCI: cdns-ep: Fix 'unevaluatedProperties' warnings

With 'unevaluatedProperties' support implemented, the TI j721e endpoint
binding example has a warning:

Documentation/devicetree/bindings/pci/ti,j721e-pci-ep.example.dt.yaml: pcie-ep@d000000: Unevaluated properties are not allowed ('max-link-speed', 'num-lanes', 'max-functions' were unexpected)

Adjust where pci-ep.yaml is referenced so that ti,j721e-pci-ep.yaml will
include it.

Cc: Tom Joseph <tjoseph@cadence.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: linux-pci@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211206194413.2469643-1-robh@kernel.org