ARM: dts: axp81x: add AC power supply subnode

Add AC power supply subnode for AXP81X PMIC.

Signed-off-by: Oskari Lemmela <oskari@lemmela.net>
Reviewed-by: Quentin Schulz <quentin.schulz@bootlin.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Tested-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>