bindings: PCI: artpec: Add support for endpoint mode

The PCIe controller integrated in ARTPEC-6 SoCs is capable of operating in
endpoint mode. Add endpoint mode support to the artpec6 driver.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Rob Herring <robh@kernel.org>