dt-bindings: regulator: Add DT binding for cros-ec-regulator

Add DT binding documentation for cros-ec-regulator, a voltage regulator
controlled by ChromeOS EC.
Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20200612040526.192878-2-pihsun@chromium.orgSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/google,cros-ec-regulator.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/regulator/google,cros-ec-regulator.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ChromeOS EC controlled voltage regulators
+
+maintainers:
+  - Pi-Hsun Shih <pihsun@chromium.org>
+
+description:
+  Any property defined as part of the core regulator binding, defined in
+  regulator.yaml, can also be used.
+
+allOf:
+  - $ref: "regulator.yaml#"
+
+properties:
+  compatible:
+    const: google,cros-ec-regulator
+
+  reg:
+    maxItems: 1
+    description: Identifier for the voltage regulator to ChromeOS EC.
+
+required:
+  - compatible
+  - reg
+
+examples:
+  - |
+    spi0 {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        cros_ec: ec@0 {
+            compatible = "google,cros-ec-spi";
+            reg = <0>;
+            #address-cells = <1>;
+            #size-cells = <0>;
+
+            regulator@0 {
+                compatible = "google,cros-ec-regulator";
+                regulator-min-microvolt = <1800000>;
+                regulator-max-microvolt = <3300000>;
+                reg = <0>;
+            };
+        };
+    };
+...