arm64: dts: rockchip: add spdif sound node for rock64

This patch adds sound card node for rock64. Currently we can support
S/PDIF only. It seems the lack of codec driver of rk3328 to enable
analog audio out.

Signed-off-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>