usb: device_next: uac2: devicetree descriptor macros

Initial set of macros to generate class specific descriptors based on
devicetree nodes. Descriptors are generated as uint8_t array initializer
list because there is variable number of entities in between standard
USB interface and endpoint descriptors. Because the descriptors are
automatically generated there is no real point in trying to generate
corresponding C structures (especially that some entity descriptors have
variably sized fields in the middle of descriptor, e.g. Clock Selector).

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