dt-bindings: display: add data-mapping to panel-dpi

Add data-mapping property that can be used to specify
the media format used for the connection betwwen the
display controller (connector) and the panel.

v2:
  - drop lvds666 (Rob)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200216181513.28109-5-sam@ravnborg.org