gpio: mention in DT binding doc that <name>-gpio is deprecated

The gpiolib supports parsing DT properties of the form <name>-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:

dd34c37aa3e8 ("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 this clear in the GPIO DT binding doc.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Rob Herring <robh@kernel.org>