Add a SPDIF audio-graph-card to ROCK Pi 4 device tree.

It's not enabled by default since all dma channels are used by
the (already) enabled i2s0/1/2 and the pin is muxed with GPIO4_C5
which might be in use already.
If enabled SPDIF_TX will be available at pin #15.
Signed-off-by: Alex Bee <knaerzche@gmail.com>
Link: https://lore.kernel.org/r/20210618181256.27992-6-knaerzche@gmail.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>