of: thermal: Mark cooling-*-level properties optional

of: thermal: Mark cooling-*-level properties optional

The cooling-{min,max}-level properties are marked as optional in
Documentation/devicetree/bindings/cpufreq/cpufreq-dt.txt and the usage
in various device tree matches this, i.e., some cooling device in the
device trees provide these properties while others do not.

Make the bindings in
Documentation/devicetree/bindings/thermal/thermal.txt consistent with
the cpufreq-dt bindings by marking the cooling-*-level properties as
optional.

Signed-off-by: Punit Agrawal <[email protected]>
Cc: Eduardo Valentin <[email protected]>
Cc: Rob Herring <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Ian Campbell <[email protected]>
Cc: Kumar Gala <[email protected]>
Signed-off-by: Rob Herring <[email protected]>