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: Iae7b1b60c1955f6590764f3de459a32320eba448
Signed-off-by: Quentin Perret <quentin.perret@arm.com>