[PATCH] doc: gpio.txt describes open-drain emulation

Update the GPIO docs to describe the idiom whereby open drain signals are
emulated by toggling the GPIO direction.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>