drm/panel: Add DT bindings for Samsung S6D16D0

This adds the device tree bindings for the Samsung S6D16D0
panel. This is a command mode only panel using DSI.

Cc: devicetree@vger.kernel.org
Cc: Andrzej Hajda <a.hajda@samsung.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181023074630.29186-1-linus.walleij@linaro.org