Input: add binding documentation for imx25 touchscreen controller

This is the touchscreen conversion queue binding documentation. It uses
the shared imx25 ADC.
Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/input/touchscreen/fsl-mx25-tcq.txt

+Freescale mx25 TS conversion queue module
+
+mx25 touchscreen conversion queue module which controls the ADC unit of the
+mx25 for attached touchscreens.
+
+Required properties:
+ - compatible: Should be "fsl,imx25-tcq".
+ - reg: Memory range of the device.
+ - interrupts: Should be the interrupt number associated with this module within
+   the tscadc unit (<0>).
+ - interrupt-parent: Should be a phandle to the tscadc unit.
+ - fsl,wires: Should be '<4>' or '<5>'
+
+Optional properties:
+ - fsl,pen-debounce-ns: Pen debounce time in nanoseconds.
+ - fsl,pen-threshold: Pen-down threshold for the touchscreen. This is a value
+   between 1 and 4096. It is the ratio between the internal reference voltage
+   and the measured voltage after the plate was precharged. Resistence between
+   plates and therefore the voltage decreases with pressure so that a smaller
+   value is equivalent to a higher pressure.
+ - fsl,settling-time-ns: Settling time in nanoseconds. The settling time is before
+   the actual touch detection to wait for an even charge distribution in the
+   plate.
+
+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:
+	tsc: tcq@50030400 {
+		compatible = "fsl,imx25-tcq";
+		reg = <0x50030400 0x60>;
+		interrupt-parent = <&tscadc>;
+		interrupts = <0>;
+		fsl,wires = <4>;
+	};