dt-bindings: opp: opp-v2-kryo-cpu: Document named opp-microvolt property

Document named opp-microvolt property for opp-v2-kryo-cpu schema.
This property is used to declare multiple voltage ranges selected on the
different values read from efuses. The selection is done based on the
speed pvs values and the named opp-microvolt property is selected by the
qcom-cpufreq-nvmem driver.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>