dt-bindings: power: supply: add AXP20X/AXP22X AC power supply

The X-Powers AXP20X and AXP22X PMICs have an AC entry to supply power to
the board. They have a few registers dedicated to the status of the AC
power supply.

This adds the DT binding documentation for the AC power supply for
AXP20X and AXP22X PMICs.
Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>

Documentation/devicetree/bindings/power/supply/axp20x_ac_power.txt

+AXP20X and AXP22X PMICs' AC power supply
+
+Required Properties:
+ - compatible: One of:
+			"x-powers,axp202-ac-power-supply"
+			"x-powers,axp221-ac-power-supply"
+
+This node is a subnode of the axp20x PMIC.
+
+The AXP20X can read the current current and voltage supplied by AC by
+reading ADC channels from the AXP20X ADC.
+
+The AXP22X is only able to tell if an AC power supply is present and
+usable.
+
+Example:
+
+&axp209 {
+	ac_power_supply: ac-power-supply {
+		compatible = "x-powers,axp202-ac-power-supply";
+	};
+};