regulator: dt-bindings: Convert pwm-regulator to DT schema

Convert the pwm-regulator binding to DT schema format.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220606184310.1057797-1-robh@kernel.org


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