dt-bindings: gpio: ds4520: Add ADI DS4520

Add ADI DS4520 devicetree document.

Signed-off-by: Okan Sahin <okan.sahin@analog.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>