dt-bindings: power: Add Spreadtrum SC27XX fuel gauge unit documentation

This patch adds the binding documentation for Spreadtrum SC27XX series PMICs
fuel gauge unit device, which is used to calculate the battery capacity.
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

Documentation/devicetree/bindings/power/supply/sc27xx-fg.txt

+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 channel to get temperature.
+- io-channel-names: Should be "bat-temp".
+- monitored-battery: Phandle of battery characteristics devicetree node.
+  See Documentation/devicetree/bindings/power/supply/battery.txt
+
+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";
+			monitored-battery = <&bat>;
+		};
+	};