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

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
v2 --> v3
- avoid awkard examples based on _cpu/_gpu regulators
v1 --> v2
- removed any reference to negative voltages
---
 .../devicetree/bindings/arm/arm,scmi.txt      | 42 +++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/Documentation/devicetree/bindings/arm/arm,scmi.txt b/Documentation/devicetree/bindings/arm/arm,scmi.txt
index 55deb68230eb..0cef83a60f03 100644
--- a/Documentation/devicetree/bindings/arm/arm,scmi.txt
+++ b/Documentation/devicetree/bindings/arm/arm,scmi.txt

@@ -62,6 +62,28 @@ Required properties:
  - #power-domain-cells : Should be 1. Contains the device or the power
 			 domain ID value used by SCMI commands.
 
+Regulator bindings for the SCMI Regulator based on SCMI Message Protocol
+------------------------------------------------------------
+
+An SCMI Regulator is permanently bound to a well defined SCMI Voltage Domain,
+and should be always positioned as a root regulator.
+It does not support any current operation.
+
+This binding uses the common regulator binding[6].
+
+SCMI Regulators are grouped under a 'regulators' node which in turn is a child
+of the SCMI Voltage protocol node inside the desired SCMI instance node.
+
+Required properties:
+ - reg : shall identify an existent SCMI Voltage Domain.
+
+Optional properties:
+ - all of the other standard regulator bindings as in [6]: note that, since
+   the SCMI Protocol itself aims in fact to hide away many of the operational
+   capabilities usually exposed by the properties of a standard regulator,
+   most of the usual regulator bindings could have just no effect in the
+   context of this SCMI regulator.

+
 Sensor bindings for the sensors based on SCMI Message Protocol
 --------------------------------------------------------------
 SCMI provides an API to access the various sensors on the SoC.

@@ -105,6 +127,7 @@ Required sub-node properties:
 [3] Documentation/devicetree/bindings/thermal/thermal*.yaml
 [4] Documentation/devicetree/bindings/sram/sram.yaml
 [5] Documentation/devicetree/bindings/reset/reset.txt
+[6] Documentation/devicetree/bindings/regulator/regulator.yaml
 
 Example:
 

@@ -169,6 +192,25 @@ firmware {
 			reg = <0x16>;
 			#reset-cells = <1>;
 		};
+
+		scmi_voltage: protocol@17 {
+			reg = <0x17>;
+
+			regulators {
+				regulator_devX: regulator_scmi_devX@0 {