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.comReviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

Documentation/devicetree/bindings/arm/arm,scmi.txt

@@ -62,6 +62,29 @@ 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.
+
+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.
+
+This binding uses the common regulator binding[6] but, due to SCMI abstractions,
+supports only a subset of its properties as specified below amongst Optional
+properties.
+
+Required properties:
+ - reg : shall identify an existent SCMI Voltage Domain.
+
+Optional properties:
+ - regulator-name
+ - regulator-min-microvolt / regulator-max-microvolt
+ - regulator-always-on / regulator-boot-on
+ - regulator-max-step-microvolt
+ - regulator-coupled-with / regulator-coupled-max-spread
+
 Sensor bindings for the sensors based on SCMI Message Protocol
 --------------------------------------------------------------
 SCMI provides an API to access the various sensors on the SoC.
@@ -105,6 +128,7 @@ 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 +193,25 @@ firmware {
 			reg = <0x16>;
 			#reset-cells = <1>;
 		};
+
+		scmi_voltage: protocol@17 {
+			reg = <0x17>;
+
+			regulators {
+				regulator_devX: regulator@0 {
+					reg = <0x0>;
+					regulator-max-microvolt = <3300000>;
+				};
+
+				regulator_devY: regulator@9 {
+					reg = <0x9>;
+					regulator-min-microvolt = <500000>;
+					regulator-max-microvolt = <4200000>;
+				};
+
+				...
+			};
+		};
 	};
 };