firmware: arm_scmi: Augment SMC/HVC to allow optional interrupt

The SMC/HVC SCMI transport is modified to allow the completion of an
SCMI message to be indicated by an interrupt rather than the return of
the smc/hvc call. This accommodates the existing behavior of the
BrcmSTB SCMI "platform".

Link: https://lore.kernel.org/r/20201222145603.40192-3-jim2101024@gmail.com


Signed-off-by: Jim Quinlan <jim2101024@gmail.com>
[sudeep.holla: added call to reinit_completion, whitespace cleanup, dropped
 irrelavant info in the commit log]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>