Documentation: gpio: mention that <function>-gpio has been deprecated

The gpiolib supports parsing DT properties of the form <function>-gpio
but it was only added for compatibility with older DT bindings that got
it wrong and should not be used in newer bindings.

The commit that added support for this was:

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

but didn't update the documentation to explain this so it's been a source
of confusion. So let's make clear this in the GPIO documentation.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>