pinctrl: axp209: add programmable ADC muxing value

To prepare for patches that will add support for a new PMIC that has a
different GPIO adc muxing value, add an adc_mux 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>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>