arm64: dts: renesas: cat874: Add HDMI video support

The CAT874 board comes with a HDMI connector, managed by
a TDA19988BET chip, connected to the RZ/G2E SoC via DPAD.
This patch adds the necessary support to the board DT.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>