ARM: tegra: nexus7: Improve thermal zones

Remove unused thermal zone just to clean up device-tree and set critical
temperature further apart from the passive cooling trip point since
during or thermal testing of Asus Transformer devices we found that CPU
could reach the critical temperature in a certain kernel configurations
for a brief moment if critical trip point is set close to the passive
trip point and then device will be immediately shut off without getting
a chance to cool down using passive cooling.

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