regulator: Add DT support for regulator-output connectors

regulator: Add DT support for regulator-output connectors

Merge series from Zev Weiss <zev@bewilderbeest.net>:

This series adds support for userspace-controlled regulator-supplied
power outputs [2].  This is an important feature for some kinds of BMC
(baseboard management controller) systems where the BMC provides an
operator with manual control of a set of DC power outputs.

As in a broadly similar patchset that was recently almost merged [0],
this takes the approach of providing support by extending the existing
userspace-consumer regulator driver.  A couple questions about the
userspace-consumer driver came up along the way, however.

First, how (if at all) is it currently being used?  It appears the
last in-tree use of it was removed a bit over two years ago in commit
9d3239147d6d ("ARM: pxa: remove Compulab pxa2xx boards").  Aside from
just adding DT support I've made a couple small tweaks to the driver
in patch 3 that I hope are compatible with any current usage, but
without any extant examples to look at it's kind of hard to say.

Second, how critical is its support for controlling multiple
regulators?  (i.e. its use of regulator_bulk_data and friends instead
of a single struct regulator.)  As far as I can see every in-tree use
of it that's ever existed has used num_supplies = 1.  If it's not
important to retain, patch 1 of this series could be supplanted by one
that instead simplifies the driver slightly by removing that
functionality.

The DT binding added in patch 2 is essentially identical to one I
posted in a previous patchset that had an R-B from Rob [1], but has
had some minor rewording and been moved from the extcon subsystem to
the regulator subsystem.