cpuidle: tegra: Support CPU cluster power-down state on Tegra30

The new Tegra CPU Idle driver now has a unified code path for the coupled
CC6 (LP2) state, this allows to enable the deepest idling state on Tegra30
SoC where the whole CPU cluster is power-gated.

Tested-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Tested-by: Jasper Korten <jja2000@gmail.com>
Tested-by: David Heidelberg <david@ixit.cz>
Tested-by: Peter Geis <pgwipeout@gmail.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>