dt-bindings: gpio: Document the Xylon LogiCVC GPIO controller

The Xylon LogiCVC display controller exports some GPIOs, which are
exposed as a separate entity.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191203141243.251058-4-paul.kocialkowski@bootlin.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>