thermal: Fix examples in DT documentation

There are various issues with the examples in this documentation, some
of the DT labels are invalid and one of the macro THERMAL_NO_LIMITS
referenced is not available as well.

This patch attempts to fix such errors in the documentation.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>