PM: EM: Add update_power() callback for runtime modifications

The Energy Model (EM) is going to support runtime 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>