[RFC][DON'T MERGE] PM: EM: Add update_power() callback

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Important Note:
This is an experimental patch aiming to discuss/test the idea of
runtime modifiable EM. It tries to address the impact of static power
(leakage) change in time due to different temperature of the chip.
The purpose of sharing this patch is to get feedback from Vendors
making the real SoCs, and gather details such as:
- does it address the problem
- doesn't contain design restrictions (such as values scale, math
  model),
- performs fast&smooth (lack of unacceptable delays in runtime updates)
- fits into the right running context of the EM modifier (e.g. thermal
governor, driver IRQ handler, etc)
- allows for optimizations and/or differentiation of the power vs.
  performance policy
After positive feedback and results the goal is to share this work
on the Linux Kernel Mailing List (together with justification) to
make it available in upstream/Android/production kernels.
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The Energy Model (EM) is going to support run-time modifications. This
new callback would be used in the upcoming EM changes. The drivers
or frameworks which might want to modify the EM due to e.g. temperature
which increases static power (leakage), could implement this callback
and call EM API at proper time to update the power values.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>