dt-bindings: usb: initial USB Audio Class 2 support

USB Audio Class 2 (UAC2) includes a method to describe audio device
topology to host using a set of class specific descriptors. The audio
device description includes complete sample clock topology and audio
processing organization.

Zephyr specific bindings are supposed to allow user to create reasonably
simple audio device description using devicetree syntax. The bindings
currently include only the absolute minimum set required for headset
example. Bindings for other entities (Clock Selector, Clock Multiplier,
Mixer Unit, Selector Unit, Feature Unit, Sample Rate Converter,
variuos Effect Units, various Processing Units, Extension Unit) can be
added later together with the actual USB class implementation.

The main idea is that user does create one zephyr,uac2 compatible node
for every USB Audio 2 class instance. Note that in majority of cases
just one USB Audio 2 class is necessary because the number of streaming
interfaces is virtually unlimited (USB Audio 2 class can have up to 255
entities). The zephyr,uac2 node includes child nodes with compatibles
set to desired entity or audiostreaming interface. The parent-child
relationship is necessary to allow grouping entities to correct audio
class instance.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>