firmware: arm_scmi: Port systempower protocol to new protocols interface

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.com


Tested-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>