Add dt-bindings documentation for Raydium RM67191 DSI panel.

Signed-off-by: Robert Chiras <robert.chiras@nxp.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1561550907-9733-2-git-send-email-robert.chiras@nxp.com