firmware: arm_scmi: Add power_scale_mw_get() interface

Add a new interface to the existing perf_ops and export the information
about the power values scale.

This would be used by the cpufreq driver and Energy Model framework to
set the performance domains scale: milli-Watts or abstract scale.

Suggested-by: Morten Rasmussen <morten.rasmussen@arm.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

include/linux/scmi_protocol.h

@@ -121,6 +121,7 @@ struct scmi_perf_ops {
 			     unsigned long *rate, unsigned long *power);
 	bool (*fast_switch_possible)(const struct scmi_handle *handle,
 				     struct device *dev);
+	bool (*power_scale_mw_get)(const struct scmi_handle *handle);
 };
 
 /**