ARM: tegra: cpuidle: Add missing cpuidle.h include

Some of the functions implemented are flagged as not having a prototype
defined when building with W=1. Include the header to avoid these build
warnings and add a prototype as well as a dummy implementation for the
tegra_cpuidle_pcie_irqs_in_use() function.

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