firmware: arm_scmi: Make smc transport use common completions

When a completion irq is available use it and delegate command completion
handling to the core SCMI completion mechanism.

If no completion irq is available revert to polling, using the core common
polling machinery.

Link: https://lore.kernel.org/r/20211220195646.44498-3-cristian.marussi@arm.com


Reviewed-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>