soc/tegra: default select TEGRA_TIMER for Tegra210

The tegra timer is necessary for Tegra210 to support CPU idle power-down
state. So select it by default.

Signed-off-by: Joseph Lo <josephl@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>