mfd: Add DT binding documentation for imx25 ADC/TSC

This documentation describes the devicetree bindings for the
ADC/Touchscreen unit of the i.MX25 SoC.
Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/fsl-imx25-tsadc.txt

+Freescale MX25 ADC/TSC MultiFunction Device (MFD)
+
+This device combines two general purpose conversion queues one used for general
+ADC and the other used for touchscreens.
+
+Required properties:
+ - compatible:			Should be "fsl,imx25-tsadc".
+ - reg:				Start address and size of the memory area of
+ 					the device
+ - interrupts:			Interrupt for this device
+					(See: ../interrupt-controller/interrupts.txt)
+ - clocks:			An 'ipg' clock (See: ../clock/clock-bindings.txt)
+ - interrupt-controller:	This device is an interrupt controller. It
+   					controls the interrupts of both
+					conversion queues.
+ - #interrupt-cells:		Should be '<1>'.
+ - #address-cells:		Should be '<1>'.
+ - #size-cells:			Should be '<1>'.
+
+This device includes two conversion queues which can be added as subnodes.
+The first queue is for the touchscreen, the second for general purpose ADC.
+
+Example:
+	tscadc: tscadc@50030000 {
+		compatible = "fsl,imx25-tsadc";
+		reg = <0x50030000 0xc>;
+		interrupts = <46>;
+		clocks = <&clks 119>;
+		clock-names = "ipg";
+		interrupt-controller;
+		#interrupt-cells = <1>;
+		#address-cells = <1>;
+		#size-cells = <1>;
+		ranges;
+
+		tsc: tcq@50030400 {
+			compatible = "fsl,imx25-tcq";
+			reg = <0x50030400 0x60>;
+			...
+		};
+
+		adc: gcq@50030800 {
+			compatible = "fsl,imx25-gcq";
+			reg = <0x50030800 0x60>;
+			...
+		};
+	};