ARM: tegra: disable LP2 cpuidle state if PCIe is enabled

Tegra20 HW appears to have a bug such that PCIe device interrupts, whether
they are legacy IRQs or MSI, are lost when LP2 is enabled. To work around
this, simply disable LP2 if the PCI driver and DT node are both enabled.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
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v3: Only disable LP2 if a PCIe device that requests an interrupt is
actually present. This prevents the loss of power-saving when the WAR
isn't required.