PCI: tegra: Add Tegra124 support

The PCIe controller on Tegra124 has two root ports that can be used in a
x4/x1 or x2/x1 configuration and can run at PCIe 2.0 link speeds (up to
5 GT/s).  The PHY programming has been moved into a separate controller, so
the driver now needs to request an external PHY referenced using the device
tree.

Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>