regulator: bindings: add MPS mp8869 voltage regulator

The MP8869 from Monolithic Power Systems is a single output dc/dc
converter with voltage control over i2c.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20200316222945.74ad34dd@xhacker


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