soc/tegra: Stub out PCIe IRQ workaround on 64-bit ARM

The PCIe host controller found on Tegra20 has a hardware bug that causes
PCIe interrupts to get lost when LP2 is enabled. Stub out the workaround
on 64-bit ARM because none of the more recent Tegra SoC generations seem
to have this bug anymore.

Signed-off-by: Thierry Reding <treding@nvidia.com>