greybus: audio: topology: Add helper API to map controlid with widget name

This API is used by enumerated controls .get/.set callback functions to
fetch control id associated with a widget.

Signed-off-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Reviewed-by: Mark Greer <mark.greer@animalcreek.com>
Signed-off-by: Alex Elder <elder@linaro.org>