firmware: arm_scmi: add a getter for power of performance states

The SCMI protocol can be used to get power estimates from firmware
corresponding to each performance state of a device. Although these power
costs are already managed by the SCMI firmware driver, they are not
exposed to any external subsystem yet.

Fix this by adding a new get_power() interface to the exisiting perf_ops
defined for the SCMI protocol.

Change-Id: I2ba654a59e9633dcb2ce4128dbd99f2317d0ece8