firmware: arm_scmi: Add support to enumerated SCMI voltage domain device

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.com
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>