Commit 4c0facdd ("gpio: core: Decouple open drain/source flag with
active low/high") decoupled the open collector outputs from active
low/high but did not update the documentation.

Update the device tree documentation to correctly reflect this new
separation between the two concepts.
Signed-off-by: NCharles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: NLaxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>