pinctrl: axp209: add programmable gpio_status_offset

To prepare for patches that will add support for a new PMIC that has a
different GPIO input status register, add a gpio_status_offset within
axp20x_pctl structure and use it.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>