firmware: arm_scmi: Port voltage protocol to new protocols interface

firmware: arm_scmi: Port voltage 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, while keeping the old handle->voltage_ops still
around to ease transition.

Remove handle->voltage_priv now unused.

Link: https://lore.kernel.org/r/20210316124903.35011-29-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>