PCI: tegra: Use generic accessors where possible

The Tegra PCI host controller can generate configuration space accesses
with byte, word and dword granularity for devices. Only root ports can't
have their configuration space accessed in this way.

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