firmware: arm_scmi: Use common iterators in the clock protocol

Make SCMI clock protocol use the common iterator protocol helpers for
issuing the multi-part commands.

Link: https://lore.kernel.org/r/20220330150551.2573938-17-cristian.marussi@arm.com


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