gpiolib: discourage gpiochip_add_pin[group]_range for DT pinctrls

This patch adds the stern warning to the kerneldoc text of both
gpiochip_add_pin[group]_range() functions in hope of detering
developers from ever using them in their DeviceTree-supported
pinctrl drivers in the future.

For anyone affected: Please refer to Section 2.1 of
Documentation/devicetree/bindings/gpio/gpio.txt on how to
bind pinctrl and gpio drivers via the "gpio-ranges" property.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>