Document the MDIO I/O voltage regulator exposed by the MxL86211C PHY.

The regulator configures the MDIO, MDC, and MDINT pad voltage
between 1.8 V and 3.3 V.

Restrict the regulator subnode to the MxL86211C PHY ID and document
the board requirement for VDDP to use the selected voltage.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>
---
 .../bindings/net/maxlinear,gpy2xx.yaml        | 40 +++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/Documentation/devicetree/bindings/net/maxlinear,gpy2xx.yaml b/Documentation/devicetree/bindings/net/maxlinear,gpy2xx.yaml
index 8a3713abd1ca9..0645e885f1747 100644
--- a/Documentation/devicetree/bindings/net/maxlinear,gpy2xx.yaml
+++ b/Documentation/devicetree/bindings/net/maxlinear,gpy2xx.yaml
@@ -12,6 +12,29 @@ maintainers:
 
 allOf:
   - $ref: ethernet-phy.yaml#
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: ethernet-phy-idc133.5400
+      required:
+        - compatible
+
+    then:
+      properties:
+        mdio-regulator:
+          type: object
+          description: |
+            Initial data for the MxL86211C MDIO I/O voltage regulator. Set
+            this to 1.8 V or 3.3 V to configure the MDIO, MDC, and MDINT
+            pad voltage. The board must supply the VDDP pin at the selected
+            voltage.
+          $ref: /schemas/regulator/regulator.yaml#
+          unevaluatedProperties: false
+
+    else:
+      properties:
+        mdio-regulator: false
 
 properties:
   maxlinear,use-broken-interrupts:
@@ -45,4 +68,21 @@ examples:
         };
     };
 
+  - |
+    ethernet {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        ethernet-phy@0 {
+            compatible = "ethernet-phy-idc133.5400",
+                         "ethernet-phy-ieee802.3-c45";
+            reg = <0>;
+
+            mdio: mdio-regulator {
+                regulator-min-microvolt = <1800000>;
+                regulator-max-microvolt = <1800000>;
+            };
+        };
+    };
+
 ...
-- 
2.55.0