firmware: arm_scmi: Make VirtIO transport a standalone driver

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

CC: Michael S. Tsirkin <mst@redhat.com>
CC: Igor Skalkin <igor.skalkin@opensynergy.com>
CC: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Message-Id: <20240812173340.3912830-9-cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>