firmware: arm_scmi: Add support for standalone transport drivers

Extend the core SCMI stack with structures and methods to allow for
transports to be split out as standalone drivers, while still supporting
old style transports, defined as built into the SCMI core stack.

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: <20240812173340.3912830-5-cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>