regulator: dt-bindings: mps,mpq7920: drop incorrect ref to regulator.yaml

"regulators" node is just grouping regulators, but itself is not
describing one regulator, thus reference to regulator.yaml schema is
incorrect.
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230725105421.99160-2-krzysztof.kozlowski@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/mps,mpq7920.yaml

@@ -21,7 +21,6 @@ properties:
 
   regulators:
     type: object
-    $ref: regulator.yaml#
 
     description: |
       list of regulators provided by this controller, must be named