PCI: tegra: Add debugfs support

Provide a debugfs file ("pcie/ports") that shows the current link status
for each root port.

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