gpio: add DT bindings for existing consumer flags

It is customary for GPIO controllers to support open drain/collector
and open source/emitter configurations. Add standard GPIO line flags
to account for this and augment the documentation to say that these
are the most generic bindings.

Several people approached me to add new flags to the lines, and this
makes sense, but let's first bind up the most common cases before we
start to add exotic stuff.

Thanks to H. Nikolaus Schaller for ideas on how to encode single-ended
wiring such as open drain/source and open collector/emitter.

Cc: Tony Lindgren <tony@atomide.com>
Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Cc: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>