dt-bindings: mfd: bd9571mwv: Convert to json-schema

Convert the ROHM BD9571MWV/BD9574MWF Power Management Integrated Circuit
(PMIC) Device Tree binding documentation to json-schema.

Make the "regulators" subnode optional, as not all users describe the
regulators.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/76fdd209e6a2dada7ff50b8ad03eb14e7f3547a6.1635338031.git.geert+renesas@glider.be