firmware: arm_scmi: Make polling mode optional

Add a check for the presence of .poll_done transport operation so that
transports that do not need to support polling mode have no need to provide
a dummy .poll_done callback either and polling mode can be disabled in the
SCMI core for that tranport.

Link: https://lore.kernel.org/r/20210803131024.40280-8-cristian.marussi@arm.com


Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>