Elantech I2C Touchscreen

Required properties:
- compatible: must be "elan,ekth3500".
- reg: I2C address of the chip.
- interrupt-parent: a phandle for the interrupt controller (see interrupt
  binding[0]).
- interrupts: interrupt to which the chip is connected (see interrupt
  binding[0]).

Optional properties:
- wakeup-source: touchscreen can be used as a wakeup source.
- pinctrl-names: should be "default" (see pinctrl binding [1]).
- pinctrl-0: a phandle pointing to the pin settings for the device (see
  pinctrl binding [1]).

[0]: Documentation/devicetree/bindings/interrupt-controller/interrupts.txt
[1]: Documentation/devicetree/bindings/pinctrl/pinctrl-bindings.txt

Example:
	&i2c1 {
		/* ... */

		touchscreen@10 {
			compatible = "elan,ekth3500";
			reg = <0x10>;
			interrupt-parent = <&gpio4>;
			interrupts = <0x0 IRQ_TYPE_EDGE_FALLING>;
			wakeup-source;
		};

		/* ... */
	};
