regulator: add support for MP8869 regulator

The MP8869 from Monolithic Power Systems is a single output DC/DC
converter. The voltage can be controlled via I2C.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20200316223020.1a6d92ae@xhacker


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