Documentation: gpio: Explain that <function>-gpio is also supported

The GPIO documentation mentions that GPIOs are mapped by defining a
<function>-gpios property in the consumer device's node but a -gpio
sufix is also supported after commit:

dd34c37a ("gpio: of: Allow -gpio suffix for property names")

Update the documentation to match the implementation.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>