Atmel maXTouch touchscreen/touchpad

Required properties:
- compatible:
    atmel,maxtouch

- reg: The I2C address of the device

- interrupts: The sink for the touchpad's IRQ output
    See ../interrupt-controller/interrupts.txt

Optional properties for main touchpad device:

- linux,gpio-keymap: An array of up to 4 entries indicating the Linux
    keycode generated by each GPIO. Linux keycodes are defined in
    <dt-bindings/input/input.h>.

- linux,gpio-keymap: When enabled, the SPT_GPIOPWN_T19 object sends messages
    on GPIO bit changes. An array of up to 8 entries can be provided
    indicating the Linux keycode mapped to each bit of the status byte,
    starting at the LSB. Linux keycodes are defined in
    <dt-bindings/input/input.h>.

    Note: the numbering of the GPIOs and the bit they start at varies between
    maXTouch devices. You must either refer to the documentation, or
    experiment to determine which bit corresponds to which input. Use
    KEY_RESERVED for unused padding values.

Example:

	touch@4b {
		compatible = "atmel,maxtouch";
		reg = <0x4b>;
		interrupt-parent = <&gpio>;
		interrupts = <TEGRA_GPIO(W, 3) IRQ_TYPE_LEVEL_LOW>;
	};