arm64: dts: rockchip: add SPDIF node for rk3399-firefly

This patch adds the SPDIF sound node and related settings
for rk3399-firefly.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Link: https://lore.kernel.org/r/20210613215237.830160-1-pbrobinson@gmail.com


Signed-off-by: Heiko Stuebner <heiko@sntech.de>