dt-bindings: pci: Document the NVIDIA Tegra264 PCIe controller

dt-bindings: pci: Document the NVIDIA Tegra264 PCIe controller

The six PCIe controllers found on Tegra264 are of two types: one is used
for the internal GPU and therefore is not connected to a UPHY and the
remaining five controllers are typically routed to a PCI slot and have
additional controls for the physical link.

While these controllers can be switched into endpoint mode, this binding
describes the root complex mode only.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>