dt-bindings: power: supply: add AXP20X/AXP22X battery DT binding

The X-Powers AXP20X and AXP22X PMICs can have a battery as power supply.

This patch adds the DT binding documentation for the battery power
supply which gets various data from the PMIC, such as the battery status
(charging, discharging, full, dead), current max limit, current current,
battery capacity (in percentage), voltage max and min limits, current
voltage and battery capacity (in Ah).
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>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>

Documentation/devicetree/bindings/power/supply/axp20x_battery.txt

+AXP20x and AXP22x battery power supply
+
+Required Properties:
+ - compatible, one of:
+			"x-powers,axp209-battery-power-supply"
+			"x-powers,axp221-battery-power-supply"
+
+This node is a subnode of the axp20x/axp22x PMIC.
+
+The AXP20X and AXP22X can read the battery voltage, charge and discharge
+currents of the battery by reading ADC channels from the AXP20X/AXP22X
+ADC.
+
+Example:
+
+&axp209 {
+	battery_power_supply: battery-power-supply {
+		compatible = "x-powers,axp209-battery-power-supply";
+	}
+};