ARM: dtsi: axp81x: add node for ADC

This adds a DT node for the ADC of the PMIC so that there can be
consumers of its IIO channels declaring their consumptions via DT.

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