ARM: tegra: cpuidle: use init/exit common routine for tegra3

Remove the duplicated code and use the cpuidle common code for initialization.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>