AXP20x and AXP22x battery power supply

Required Properties:
 - compatible, one of:
			"x-powers,axp209-battery-power-supply"
			"x-powers,axp221-battery-power-supply"
			"x-powers,axp813-battery-power-supply"

This node is a subnode of its respective PMIC DT node.

The supported devices can read the battery voltage, charge and discharge
currents of the battery by reading ADC channels from the ADC.

Example:

&axp209 {
	battery_power_supply: battery-power-supply {
		compatible = "x-powers,axp209-battery-power-supply";
	}
};
