OPP: Add support of "opp-microwatt" for EM registration

The Energy Model (EM) can be created based on DT entry:
'dynamic-power-coefficient'. It's a 'simple' EM which is limited to the
dynamic power. It has to fit into the math formula which requires also
information about voltage. Some of the platforms don't expose voltage
information, thus it's not possible to use EM registration using DT.

This patch aims to fix it. It introduces new implementation of the EM
registration callback. The new mechanism relies on the new OPP feature
allowing to get power (which is coming from "opp-microwatt" DT property)
expressed in micro-Watts.

The patch also opens new opportunity to better support platforms, which
have a decent static power. It allows to register the EM based on real
power measurements which models total power (static + dynamic), so better
reflects real HW.

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