firmware: arm_scmi: Make SMC transport a standalone driver

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

CC: Peng Fan <peng.fan@nxp.com>
CC: Nikunj Kela <quic_nkela@quicinc.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Message-Id: <20240812173340.3912830-7-cristian.marussi@arm.com>
[sudeep.holla: moved Clang Thumb2 build fix to the new makefile]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>