of: add notes of critical trips for soctherm

The "critical" type trip in thermal zone can be
set to SOC_THERM hardware, it can trigger shut down
or reset event from hardware.

Signed-off-by: Wei Ni <wni@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>