regulator: mp886x: Document MP8867 support

MP8867 is an I2C-controlled adjustable voltage regulator made by
Monolithic Power Systems. The difference between MP8867 and MP8869
are:
1.If V_BOOT, the vref of MP8869 is fixed at 600mv while vref of MP8867
is determined by the I2C control.
2.For MP8867, when setting voltage, if the steps is within 5, we need
to manually set the GO_BIT to 0.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20200316223100.37805b22@xhacker


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