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 <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>

Documentation/devicetree/bindings/thermal/thermal.txt

@@ -60,14 +60,6 @@ properties) should be defined in a particular device's binding.
 For more examples of cooling devices, refer to the example sections below.
 
 Required properties:
-- cooling-min-level:	An integer indicating the smallest
-  Type: unsigned	cooling state accepted. Typically 0.
-  Size: one cell
-
-- cooling-max-level:	An integer indicating the largest
-  Type: unsigned	cooling state accepted.
-  Size: one cell
-
 - #cooling-cells:	Used to provide cooling device specific information
   Type: unsigned	while referring to it. Must be at least 2, in order
   Size: one cell      	to specify minimum and maximum cooling state used
@@ -77,6 +69,15 @@ Required properties:
 			See Cooling device maps section below for more details
 			on how consumers refer to cooling devices.
 
+Optional properties:
+- cooling-min-level:	An integer indicating the smallest
+  Type: unsigned	cooling state accepted. Typically 0.
+  Size: one cell
+
+- cooling-max-level:	An integer indicating the largest
+  Type: unsigned	cooling state accepted.
+  Size: one cell
+
 * Trip points
 
 The trip node is a node to describe a point in the temperature domain