firmware: arm_scmi: Review virtio free_list handling

firmware: arm_scmi: Review virtio free_list handling

Add a new spinlock dedicated to the access of the TX free list and a couple
of helpers to get and put messages back and forth from the free_list.

Link: https://lore.kernel.org/r/20220213195832.27932-3-cristian.marussi@arm.com
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Igor Skalkin <igor.skalkin@opensynergy.com>
Cc: Peter Hilber <peter.hilber@opensynergy.com>
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>