dt-bindings: arm: Add virtio transport for SCMI

Document the properties for arm,scmi-virtio compatible nodes.
The backing virtio SCMI device is described in patch [1].

While doing that, make shmem property required only for pre-existing
mailbox and smc transports, since virtio-scmi does not need it.

[1] https://lists.oasis-open.org/archives/virtio-comment/202102/msg00018.html

Link: https://lore.kernel.org/r/20210803131024.40280-14-cristian.marussi@arm.com


Co-developed-by: Peter Hilber <peter.hilber@opensynergy.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Igor Skalkin <igor.skalkin@opensynergy.com>
[ Peter: Adapted patch for submission to upstream. ]
Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
[ Cristian: converted to yaml format, moved shmen required property. ]
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>