dt-bindings: display: Add idk-2121wr binding

Add binding for the idk-2121wr LVDS panel from Advantech.

Some panel-specific documentation can be found here:
https://buy.advantech.eu/Displays/Embedded-LCD-Kits-High-Brightness/model-IDK-2121WR-K2FHA2E.htm



Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1583869169-1006-1-git-send-email-prabhakar.mahadev-lad.rj@bp.renesas.com