ARM: tegra30: cpuidle: add powered-down state for secondary CPUs

This supports power-gated idle on secondary CPUs for Tegra30. The
secondary CPUs can go into powered-down state independently. When
CPU goes into this state, it saves it's contexts and puts itself
to flow controlled WFI state. After that, it will been power gated.

Be aware of that, you may see the legacy power state "LP2" in the
code which is exactly the same meaning of "CPU power down".

Based on the work by:
Scott Williams <scwilliams@nvidia.com>

Signed-off-by: Joseph Lo <josephl@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>