32-bit and 64-bit ARM use very different infrastructure to register a PCI
host bridge.  The Tegra PCIe host controller driver currently only supports
the 32-bit ARM infrastructure, so prevent it from being built on 64-bit ARM
where it will break.
Signed-off-by: NThierry Reding <treding@nvidia.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>