cpuidle: tegra: Disable CC6 state if LP2 unavailable

LP2 suspending could be unavailable, for example if it is disabled in a
device-tree. CC6 cpuidle state won't work in that case.

Acked-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>