dt-bindings: input: add bindings document for ar1021_i2c driver

Add a simple binding document describing the supported devices and the
I2C bus address.
Signed-off-by: Martin Kepplinger <martin.kepplinger@ginzinger.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/touchscreen/ar1021.txt

+* Microchip AR1020 and AR1021 touchscreen interface (I2C)
+
+Required properties:
+- compatible		: "microchip,ar1021-i2c"
+- reg			: I2C slave address
+- interrupt-parent	: the phandle for the interrupt controller
+- interrupts		: touch controller interrupt
+
+Example:
+
+	touchscreen@4d {
+		compatible = "microchip,ar1021-i2c";
+		reg = <0x4d>;
+		interrupt-parent = <&gpio3>;
+		interrupts = <11 IRQ_TYPE_LEVEL_HIGH>;
+	};