dt-bindings: usb: uac2: Add Feature Unit bindings

Add initial Feature Unit bindings allowing user to place the Feature
Unit inside UAC2 instance description. Currently the bindings facilitate
only specifying which controls are available on the Primary channel 0
and on each Logical channel. The number of Logical channels has to be
derived from data-source property.

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