gpio/board.txt: point to gpiod_set_value

gpiod_set_value() is preffered interface these days, so add a
pointer. Also fix a missing ).

Signed-off-by: Pavel Machek <pavel@ucw.cz>
[Fixed some grammar and reworded]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>