drm/panel: Add support for Raydium RM68200 panel driver

This patch adds Raydium Semiconductor Corporation RM68200 5.5" 720x1280
TFT LCD panel driver (MIPI-DSI video mode).

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180302153222.4377-3-philippe.cornu@st.com