arm64: dts: rockchip: Enable HDMI audio on Rock Pi

This commit enables the hdmi-sound device needed to have
audio over HDMI on the Rock Pi board.

Fixes: 1b5715c6

 ("arm64: dts: rockchip: add ROCK Pi 4 DTS support")
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>