dt-bindings: display: panel: Add support for Raydium RM68200 DSI panel

The Raydium Semiconductor Corporation RM68200 is a 5.5" 720x1280
TFT LCD panel connected using a MIPI-DSI video interface.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180302153222.4377-2-philippe.cornu@st.com