ARM: multi_v7_defconfig: Enable Tegra cpuidle driver

The Tegra CPU Idle driver was moved out into driver/cpuidle/ directory and
it is now a proper platform driver.

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>