dt-bindings: PCI: tegra234: Add schema for tegra234 Endpoint mode

Add support for PCIe controllers that operate in the Endpoint mode in
tegra234 chipset.

Link: https://lore.kernel.org/r/20220721142052.25971-4-vidyas@nvidia.com


Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>