arm64: dts: renesas: cat874: Add HDMI audio

The CAT874 board pushes sound via I2S over SSI0 into the
TDA19988BET chip.
This commit wires things up so that we can get sound out of
the HDMI interface.

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