ARM: tegra: nexus7: Improve thermal zones

Use skin temperature for maintaining temperature that is suitable
specifically for Nexus 7. Add CPU thermal zone that protects silicon.
All these changes don't make a significant difference, but it is a
more correct definition of thermal zones.

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