arm64: dts: renesas: Add EK874 board with idk-2121wr display support

arm64: dts: renesas: Add EK874 board with idk-2121wr display support

The EK874 is advertised as compatible with panel IDK-2121WR from
Advantech, however the panel isn't sold alongside the board.
A new dts, adding everything that's required to get the panel to
to work with the EK874, is the most convenient way to support the
EK874 when it's connected to the IDK-2121WR.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/1576590361-28244-7-git-send-email-fabrizio.castro@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>