gpio: of: Reflect decoupling of open collector and active low/high

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: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>