dt-bindings: firmware: imx: Add imx-scu gpio node

Add the description for imx-scu gpio subnode.

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