dt-bindings: thermal: Remove "cooling-{min|max}-level" properties

The "cooling-min-level" and "cooling-max-level" properties are not
parsed by any part of kernel currently and the max cooling state of a
CPU cooling device is found by referring to the cpufreq table instead.

Remove the unused bindings.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>