arm64: tegra: Add PCIe host bridge on Tegra210

Add the PCIe host bridge found on Tegra X1. It implements two root ports
that support x4 and x1 configurations, respectively.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>