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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

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

···
 
   regulators:
     type: object
-    $ref: regulator.yaml#
 
     description: |
       list of regulators provided by this controller, must be named