Add devicetree bindings to support regulators based on SCMI Voltage
Domain Protocol.

Link: https://lore.kernel.org/r/20201119191051.46363-5-cristian.marussi@arm.comReviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NCristian Marussi <cristian.marussi@arm.com>
Signed-off-by: NSudeep Holla <sudeep.holla@arm.com>