Add SCMI voltage domain device name to the core list of supported protocol
devices so that it can be enumerated if the firmware supports it.

Link: https://lore.kernel.org/r/20201119191051.46363-3-cristian.marussi@arm.comSigned-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>