cpufreq: arm_scmi: Discover the power scale in performance protocol

Add mechanism to discover the power scale present in the performance
protocol for all domains. Provide this information to Energy Model,
which then can be checked in other frameworks, e.g. thermal.

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