dt-bindings: gpio: Add imx scu gpio driver bindings

Add binding document for the imx scu gpio driver.

Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>