boards: arm: nrf5340_audio_dk_nrf5340: add bindings for audio codec

The board embeds an audio codec IC, Cirrus CS42L73. While there is no
in-tree driver for it, we need a binding to describe how it is wired in
the board, e.g. for IRQ/RESET lines.

Following Linux binding for the same IC, create one in
dts/bindings/sound. Note that Linux binding is less complete/outdated.

Ref. https://statics.cirrus.com/pubs/proDatasheet/CS47L63_DS1249F2.pdf



Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>