power: da9150: Add DT bindings documentation for Fuel-Gauge

Dialog Semiconductor DA9150 Fuel-Gauge Power Supply bindings

Required properties:

- compatible: "dlg,da9150-fuel-gauge" for DA9150 Fuel-Gauge Power Supply

Optional properties:

- dlg,update-interval: Interval time (milliseconds) between battery level checks.

- dlg,warn-soc-level: Battery discharge level (%) where warning event raised.

      [1 - 100]

- dlg,crit-soc-level: Battery discharge level (%) where critical event raised.

  This value should be lower than the warning level.

      [1 - 100]

Example:

	fuel-gauge {

		compatible = "dlg,da9150-fuel-gauge";

		dlg,update-interval = <10000>;

		dlg,warn-soc-level = /bits/ 8 <15>;

		dlg,crit-soc-level = /bits/ 8 <5>;

	};