document: devicetree: bind pinconf with pin single

Add comments with pinconf & gpio range in the document of
pinctrl-single.

Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>