mfd: da9150: Add DT binding documentation for core

Signed-off-by: NAdam Thomson <Adam.Thomson.Opensource@diasemi.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/da9150.txt

+Dialog Semiconductor DA9150 Combined Charger/Fuel-Gauge MFD bindings
+
+DA9150 consists of a group of sub-devices:
+
+Device			 Description
+------			 -----------
+da9150-gpadc		: General Purpose ADC
+da9150-charger		: Battery Charger
+
+======
+
+Required properties:
+- compatible : Should be "dlg,da9150"
+- reg: Specifies the I2C slave address
+- interrupt-parent: Specifies the phandle of the interrupt controller to which
+  the IRQs from da9150 are delivered to.
+- interrupts: IRQ line info for da9150 chip.
+- interrupt-controller: da9150 has internal IRQs (own IRQ domain).
+  (See Documentation/devicetree/bindings/interrupt-controller/interrupts.txt for
+   further information relating to interrupt properties)
+
+Sub-devices:
+- da9150-gpadc: See Documentation/devicetree/bindings/iio/adc/da9150-gpadc.txt
+- da9150-charger: See Documentation/devicetree/bindings/power/da9150-charger.txt
+
+
+Example:
+
+	charger_fg: da9150@58 {
+		compatible = "dlg,da9150";
+		reg = <0x58>;
+		interrupt-parent = <&gpio6>;
+		interrupts = <11 IRQ_TYPE_LEVEL_LOW>;
+		interrupt-controller;
+
+		gpadc: da9150-gpadc {
+			...
+		};
+
+		da9150-charger {
+			...
+		};
+	};