firmware: arm_scmi: Add proper barriers to scmi virtio device

Only one single SCMI Virtio device is currently supported by this driver
and it is referenced using a static global variable which is initialized
once for all during probing and nullified at virtio device removal.

Add proper SMP barriers to protect accesses to such device reference to
ensure that the initialzation state of such device is correctly observed by
all PEs at any time.

Return -EBUSY, instead of -EINVAL, and a descriptive error message if more
than one SCMI Virtio device is ever found and probed.

Link: https://lore.kernel.org/r/20210916103336.7243-3-cristian.marussi@arm.com
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>