arm64: dts: rockchip: add SPDIF node for rk3399-rockpro64

This patch adds 'disabled' SPDIF sound node and related settings
for rk3399-rockpro64.

There are 2 reasons:
  - All RK3399 dma-bus channels have been already used by I2S0/1/2
  - RockPro64 does not have SPDIF optical nor coaxial connector,
    just have 3pins

Signed-off-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>
Link: https://lore.kernel.org/r/20200810091619.3170534-1-katsuhiro@katsuster.net


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