dt-bindings: pci: tegra: Document Tegra186 PCIe DT

Tegra186 PCIe controller DT properties has couple of differences wrt
Tegra210 PCIe, rest of the DT properties are same.

Tested-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Manikanta Maddireddy <mmaddireddy@nvidia.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>