dt-bindings: arm: Add support for SCMI Regulators

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

Link: https://lore.kernel.org/r/20201119191051.46363-5-cristian.marussi@arm.com
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>