firmware: arm_scmi: Introduce packet handling helpers

Introduce a pair of structures initialized to contain all the existing
packet handling helpers, both for transports based on shared memory
and messages.

No functional change.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Tested-by: Peng Fan <peng.fan@nxp.com>  #i.MX95 19x19 EVK
Tested-by: Florian Fainelli <florian.fainelli@broadcom.com>
Message-Id: <20240812173340.3912830-4-cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>