regulator: Document "regulator-boot-on" binding more thoroughly

The description of "regulator-boot-on" was a little unclear, at least
to me.  Did this property mean that we should turn the regulator on at
boot?  Or perhaps it was intended only to be used for regulators where
we couldn't read the state at bootup to indicate what state we should
assume?  The answer, it turns out, is both [1].

Let's document this.

[1] https://lore.kernel.org/r/20190923181431.GU2036@sirena.org.uk



Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20191001124531.v2.1.Ice34ad5970a375c3c03cb15c3859b3ee501561bf@changeid


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