firmware: arm_scmi: Fix kernel doc warnings

Kernel doc validation script is unhappy and complains with the below set
of warnings.

 | Function parameter or member 'fast_switch_possible' not described in 'scmi_perf_proto_ops'
 | Function parameter or member 'power_scale_mw_get' not described in 'scmi_perf_proto_ops'
 | cannot understand function prototype: 'struct scmi_sensor_reading '
 | cannot understand function prototype: 'struct scmi_range_attrs '
 | cannot understand function prototype: 'struct scmi_sensor_axis_info '
 | cannot understand function prototype: 'struct scmi_sensor_intervals_info '

Fix them adding appropriate documents or missing keywords.

Link: https://lore.kernel.org/r/20210712130801.2436492-2-sudeep.holla@arm.com
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>