firmware: arm_scmi: Add iterators for multi-part commands

SCMI specification defines some commands as optionally issued over multiple
messages in order to overcome possible limitations in payload size enforced
by the configured underlyinng transport.

Introduce some common protocol helpers to provide a unified solution for
issuing such SCMI multi-part commands.

Link: https://lore.kernel.org/r/20220330150551.2573938-14-cristian.marussi@arm.com


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