firmware: arm_scmi: Add SCMI v3.0 sensors timestamped reads

Add new .reading_get_timestamped() method to sensor_ops to support SCMI v3.0
timestamped reads.

Link: https://lore.kernel.org/r/20201119174906.43862-5-cristian.marussi@arm.com


Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>