Some platforms may have support for offloading USB audio devices to a
dedicated audio DSP.  Introduce a set of APIs that allow for management of
USB sound card and PCM devices enumerated by the USB SND class driver.
This allows for the ASoC components to be aware of what USB devices are
available for offloading.

Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com>