dt-bindings: Add binding for Ilitek ILI9225 display panels

This adds a new binding for display panels that use an Ilitek ILI9225
controller.

The "ilitek,ili9225-2.2in-176x220" device listed has no identification
markings whatsoever and an hour of googling turned up nothing, hence the use
of the size and resolution in the name instead of a model name.

An example of this nameless device can be found at:
https://github.com/Nkawu/TFT_22_ILI9225



Signed-off-by: David Lechner <david@lechnology.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1511122328-31133-3-git-send-email-david@lechnology.com