arm64: dts: renesas: r8a774a1: Add HDMI encoder instance

Add the HDMI encoder to the R8A774A1 DT in disabled state.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>