gpio: Document ZTE zx296702 GPIO DT binding

Add document of ZTE zx296702 GPIO binding

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>