tegra-cpuidle: provide stub when !CONFIG_CPU_IDLE

While poking at something using the for-3.12/* trees, I hit the
following compile error:
drivers/built-in.o: In function `tegra_pcie_map_irq':
/builddir/build/BUILD/kernel-3.10.fc20/linux-3.11.0-0.rc6.git4.1.fc20.armv7hl/drivers/pci/host/pci-tegra.c:640:
undefined reference to `tegra_cpuidle_pcie_irqs_in_use'
drivers/built-in.o: In function `tegra_msi_map':
/builddir/build/BUILD/kernel-3.10.fc20/linux-3.11.0-0.rc6.git4.1.fc20.armv7hl/drivers/pci/host/pci-tegra.c:1227:
undefined reference to `tegra_cpuidle_pcie_irqs_in_use'
make: *** [vmlinux] Error 1

Since our .config had CONFIG_CPU_IDLE off. We should probably provide
an empty function to handle this to avoid cluttering up pci-tegra.c
with conditionals.

Signed-off-by: Kyle McMartin <kyle@redhat.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
[swarren, removed unnecessary return statement]
Signed-off-by: Stephen Warren <swarren@nvidia.com>