* <http://bugs.libre-riscv.org/show_bug.cgi?id=3>
* <https://github.com/skristiansson/i2s>
* <https://en.wikipedia.org/wiki/I%C2%B2S>
+* <http://www.ti.com/lit/an/snla221/snla221.pdf>
+* <https://www.sparkfun.com/datasheets/BreakoutBoards/I2SBUS.pdf>
Question: how does I2S do 4-wire output?