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>