dt-bindings: mfd: Convert rn5t618 to json-schema

Convert the RN5T618 binding to DT schema format. Also
clearly state which regulators are available.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>