drm/panel: Add DT bindings for Ilitek ILI9322

This adds device tree bindings for the Ilitek ILI9322
320x240 TFT panel driver.

Cc: David Lechner <david@lechnology.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Ben Dooks <ben.dooks@codethink.co.uk>
Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171201161659.9588-1-linus.walleij@linaro.org