regulator: mp8859: add driver

The MP8859 from Monolithic Power Systems is a single output DC/DC
converter. The voltage can be controlled via I2C.

Signed-off-by: Markus Reichl <m.reichl@fivetechno.de>
Link: https://lore.kernel.org/r/20200106211633.2882-2-m.reichl@fivetechno.de


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