dt-bindings: panel: Add the Ilitek ILI9881c panel documentation

The LHR050H41 from BananaPi is a 1280x700 4-lanes DSI panel based on the
ILI9881c from Ilitek.

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a348cdd07d3287e8203ee8d840ea279fe10a6204.1527587352.git-series.maxime.ripard@bootlin.com