PCI: tegra: Add Tegra 30 PCIe support

Introduce a data structure to parameterize the driver according to SoC
generation, add Tegra30 specific code and update the device tree binding
document for Tegra30 support.

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