Transceivers can both receive and provide audio, and streams can be
routed from one node to any other, including many others. The tricky
bit is how to expose the audio routing in DT in a sane way.
The way it is implemented here, the slave nodes specify the number of
slots they each consume and generate, and which thereof they forward
from one side to the other. This mimics the internal register
structure and should allow for even exotic setups.