arm64: dts: rockchip: add HDMI audio on ROCK 4D

arm64: dts: rockchip: add HDMI audio on ROCK 4D

Much like the Sige5, the ROCK 4D also has an HDMI port, so is capable of
providing HDMI audio output as well.

Enable the SoC's hdmi_sound card, and also enable the SoC audio
controller (sai6) that feeds into it.

Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Tested-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://lore.kernel.org/r/20250630-rock4d-audio-v1-4-0b3c8e8fda9c@collabora.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>