Spreadtrum SC27XX PMICs Fuel Gauge Unit Power Supply Bindings

Required properties:
- compatible: Should be one of the following:
  "sprd,sc2720-fgu",
  "sprd,sc2721-fgu",
  "sprd,sc2723-fgu",
  "sprd,sc2730-fgu",
  "sprd,sc2731-fgu".
- reg: The address offset of fuel gauge unit.
- battery-detect-gpios: GPIO for battery detection.
- io-channels: Specify the IIO ADC channels to get temperature and charge voltage.
- io-channel-names: Should be "bat-temp" and "charge-vol"
- sprd,calib-resistance-real: The real resistance of coulomb counter chip in microohms
  is used to calibrate cur_1000ma_adc.
- sprd,calib-resistance-spec: The specification resistance of coulomb counter chip in microohms
  is used to calibrate cur_1000ma_adc.
- nvmem-cells: A phandle to the calibration cells provided by eFuse device.
- nvmem-cell-names: Should be "fgu_calib".
- monitored-battery: Phandle of battery characteristics devicetree node.
  See Documentation/devicetree/bindings/power/supply/battery.txt

Optional Properties:
- sprd,comp-resistance-mohm: The coulomb counter internal resistance and the board
  ground resistance.
  Due to the ntc resistor is connected to the coulomb counter internal resistance of 20 million
  and the board ground impedance at 1850mv. so need to compensate for coulomb resistance and
  voltage loss to ground impedance.

Example:

	bat: battery {
		compatible = "simple-battery";
		charge-full-design-microamp-hours = <1900000>;
		constant-charge-voltage-max-microvolt = <4350000>;
		ocv-capacity-celsius = <20>;
		ocv-capacity-table-0 = <4185000 100>, <4113000 95>, <4066000 90>,
					<4022000 85>, <3983000 80>, <3949000 75>,
					<3917000 70>, <3889000 65>, <3864000 60>,
					<3835000 55>, <3805000 50>, <3787000 45>,
					<3777000 40>, <3773000 35>, <3770000 30>,
					<3765000 25>, <3752000 20>, <3724000 15>,
					<3680000 10>, <3605000 5>, <3400000 0>;
		......
	};

	sc2731_pmic: pmic@0 {
		compatible = "sprd,sc2731";
		reg = <0>;
		spi-max-frequency = <26000000>;
		interrupts = <GIC_SPI 31 IRQ_TYPE_LEVEL_HIGH>;
		interrupt-controller;
		#interrupt-cells = <2>;
		#address-cells = <1>;
		#size-cells = <0>;

		fgu@a00 {
			compatible = "sprd,sc2731-fgu";
			reg = <0xa00>;
			battery-detect-gpios = <&pmic_eic 9 GPIO_ACTIVE_HIGH>;
			io-channels = <&pmic_adc 5>;
			io-channel-names = "bat-temp";
			nvmem-cells = <&fgu_calib>;
			nvmem-cell-names = "fgu_calib";
			monitored-battery = <&bat>;
			sprd,calib-resistance-real = <21500>;
			sprd,calib-resistance-spec = <20000>;
			sprd,comp-resistance-mohm = <30>;
		};
	};
