dt-bindings: opp: Add "opp-microwatt" entry in the OPP

Add new entry for the OPP which provides information about power
expressed in micro-Watts. It is useful for the Energy Model framework.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>