pci: tegra: Add Tegra PCIe driver

Add support for the PCIe controller found on some generations of Tegra.
Tegra20 has 2 root ports with a total of 4 lanes, Tegra30 has 3 root
ports with a total of 6 lanes and Tegra124 has 2 root ports with a total
of 5 lanes.

This is based on the Linux kernel driver, originally submitted upstream
by Mike Rapoport.

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>