pinctrl: core: comment that pinctrl_add_gpio_range() is deprecated

The pinctrl_add_gpio_range() function is deprecated add a comment so
people don't accidentally use it in new code.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/533a7a10-c6eb-4ebe-adf1-f8dc95ae8d33@moroto.mountain


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>