regulator: dt-bindings: dlg,da9121: add buck2 constraints

Instead of describing the constraints for some devices (buck2 not
present), code it in allOf:if:then section to actually allow validation
of this requirement.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230725124629.150113-1-krzysztof.kozlowski@linaro.org


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