regulator: dt-bindings: maxim,max77686: Convert to dtschema

Convert the regulators of Maxim MAX77686 PMIC to DT schema format.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Acked-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211125074826.7947-2-krzysztof.kozlowski@canonical.com