dt-bindings: gpio: Add gpio-delay binding document

This adds bindings for a GPIO enable/disable delay driver.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>