Make SCMI OPTEE transport a standalone driver that can be optionally
loaded as a module.

CC: Etienne Carriere <etienne.carriere@foss.st.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Message-Id: <20240812173340.3912830-8-cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>