dt-bindings: new binding for Ilitek ILI9341 display panels

dt-bindings: new binding for Ilitek ILI9341 display panels

This adds a new binding for Ilitek ILI9341 display panels. It includes
a compatible string for one display (more can be added in the future).

The YX240QV29-T panel[1] is found, for example, in an Adafruit breakout
board[2] and in Mindsensors' PiStorms[3].

The vendor prefix "adafruit" is used because the actual vendor is not
known, but Adafruit is the most common source for a product that
contains this panel.

[1]: https://cdn-learn.adafruit.com/assets/assets/000/046/879/original/SPEC-YX240QV29-T_Rev.A__1_.pdf
[2]: https://www.adafruit.com/product/2478
[3]: http://www.mindsensors.com/stem-with-robotics/13-pistorms-v2-base-kit-raspberry-pi-brain-for-lego-robot

Signed-off-by: David Lechner <david@lechnology.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525193623.15533-4-david@lechnology.com