pinctrl: abx500: fix abx500_gpio_get()

- allow to get output GPIO value
- as there is no GPIO0 on ABX500, use correct offset with
  abx500_gpio_get_bit()

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>