Convert internals of protocol implementation to use protocol handles and
expose a new protocol operations interface for SCMI driver using the new
get/put common operations.

Remove handle->system_priv now unused.

Link: https://lore.kernel.org/r/20210316124903.35011-28-cristian.marussi@arm.comTested-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NCristian Marussi <cristian.marussi@arm.com>
Signed-off-by: NSudeep Holla <sudeep.holla@arm.com>