dt-bindings: regulator: mediatek,mt6331: Add missing compatible

This binding had no compatible and for this reason would not be
applied to anything: add the missing "mediatek,mt6331-regulator"
comaptible.

Fixes: 6385e21 ("regulator: Add bindings for MT6331 regulator")
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

Author: AngeloGioacchino Del Regno

Documentation/devicetree/bindings/regulator/mediatek,mt6331-regulator.yaml

@@ -15,6 +15,10 @@ description: |
   buck-<name> and ldo-<name>.
   MT6331 regulators node should be sub node of the MT6397 MFD node.
 
+properties:
+  compatible:
+    const: mediatek,mt6331-regulator
+
 patternProperties:
   "^buck-v(core2|io18|dvfs11|dvfs12|dvfs13|dvfs14)$":
     type: object
@@ -82,6 +86,9 @@ patternProperties:
 
     unevaluatedProperties: false
 
+required:
+  - compatible
+
 additionalProperties: false
 
 examples: