Add a new xfer parameter to mark_txdone transport operation which enables
the SCMI core to optionally pass back into the transport layer a reference
to the xfer descriptor that is being handled.

Link: https://lore.kernel.org/r/20211220195646.44498-9-cristian.marussi@arm.comReviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>