of: thermal: Fix inconsitency between cooling-*-state and cooling-*-level

The device trees in the kernel as well as the binding description in
Documentation/devicetree/bindings/cpufreq/cpufreq-dt.txt use the
cooling-{min,max}-level property.

Fix the inconsistency with the binding description in
Documentation/devicetree/bindings/thermal/thermal.txt by changing
cooling-*-state properties to cooling-*-level.

Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Rob Herring <robh@kernel.org>