regulator: Convert regulator binding to json-schema

Convert the common regulator binding to DT schema format. Note that all
the properties with standard unit suffixes have type checks already, so
only a description is necessary.

As fixed-regulator has already been converted, update the references in
it. Otherwise, keep regulator.txt with a reference to the schema to
avoid a bunch of treewide updates. regulator.txt can be removed when all
the regulator bindings are converted.

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