gpio: document interaction with other subsystems

Now I am very fed up with people reinventing kernel wheels in
userspace "just because they can" (read, sysfs). Put in a angry
blurb in sysfs doc and put in a new file with pointers to other
subsystem drivers utilizing GPIOs.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>