pinctrl: st: use new pinctrl GPIO helpers

Replace the pinctrl helpers taking the global GPIO number as argument
with the improved variants that instead take a pointer to the GPIO chip
and the controller-relative offset.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>