arm64: tegra: Add missing gpu-throt-level to Tegra210 soctherm

On Jetson TX1 the following message can be seen:

 tegra_soctherm 700e2000.thermal-sensor: throttle-cfg: heavy: no throt prop or invalid prop

This patch will fix the invalid prop issue according to the binding.

Signed-off-by: Nicolas Chauvet <kwizart@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>