ARM: tegra_defconfig: Enable CONFIG_DEVFREQ_THERMAL

Memory-related Tegra devfreq devices now could be used as a cooling
devices. Enable CONFIG_DEVFREQ_THERMAL by default since this option
enables cooling functionality of the devfreq drivers.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>