ARM: tegra: Add Tegra20 PCIe support to DT

Add the top-level pcie-controller node for the Tegra20 SoC. Tegra20 has
two root ports that can use different lane layouts.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
[swarren: split DT changes into a separate patch from the main driver]
Signed-off-by: Stephen Warren <swarren@nvidia.com>