pinctrl: abx500: use gpiochip_dup_line_label()

Use the new gpiochip_dup_line_label() helper to safely retrieve the
descriptor label.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>