dt-bindings: input: touchscreen: add max11801-ts binding

Add missing documentation of max11801-ts dt-binding details.
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/touchscreen/max11801-ts.txt

+* MAXI MAX11801 Resistive touch screen controller with i2c interface
+
+Required properties:
+- compatible: must be "maxim,max11801"
+- reg: i2c slave address
+- interrupt-parent: the phandle for the interrupt controller
+- interrupts: touch controller interrupt
+
+Example:
+
+&i2c1 {
+	max11801: touchscreen@48 {
+		compatible = "maxim,max11801";
+		reg = <0x48>;
+		interrupt-parent = <&gpio3>;
+		interrupts = <31 IRQ_TYPE_EDGE_FALLING>;
+	};
+};