The X-Powers AXP22X PMIC exposes the status of AC power supply.

This adds the AC power supply subnode for the AXP22X PMIC.
Signed-off-by: NQuentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: NChen-Yu Tsai <wens@csie.org>
Acked-by: NMaxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: NMaxime Ripard <maxime.ripard@free-electrons.com>