regulator: bindings: add MPS mp8859 voltage regulator

The MP8859 from Monolithic Power Systems is a single output dc/dc converter
with voltage control over i2c.

Signed-off-by: Markus Reichl <m.reichl@fivetechno.de>
Link: https://lore.kernel.org/r/20200106211633.2882-5-m.reichl@fivetechno.de


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