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.org


Signed-off-by: Mark Brown <broonie@kernel.org>