dt-bindings: regulator: Convert mp886x to json-schema

Convert the mp886x binding to DT schema format using json-schema.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200827150640.267f6edc@xhacker.debian


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