ARM: dtsi: axp209: add AC power supply subnode

The X-Powers AXP20X PMIC exposes the status of AC power supply, the
current current and voltage supplied to the board by the AC power
supply.

This adds the AC power supply subnode for AXP20X PMIC.

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