PM / OPP: dt-bindings: Make "opp-hz" optional for power domains

The "opp-hz" property is not relevant across all the devices that use
the OPP tables now. For example, for a power domain a frequency value
wouldn't mean anything. Though they must have another property, which
may be implementation defined, which uniquely identifies the OPP nodes.

Make "opp-hz" optional for such devices.
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/opp/opp.txt

@@ -82,7 +82,10 @@ This defines voltage-current-frequency combinations along with other related
 properties.
 
 Required properties:
-- opp-hz: Frequency in Hz, expressed as a 64-bit big-endian integer.
+- opp-hz: Frequency in Hz, expressed as a 64-bit big-endian integer. This is a
+  required property for all device nodes but devices like power domains. The
+  power domain nodes must have another (implementation dependent) property which
+  uniquely identifies the OPP nodes.
 
 Optional properties:
 - opp-microvolt: voltage in micro Volts.