greybus: audio: topology: Enable enumerated control support

Added .get/.set callback and relevant changes in parser to enable
enumerated control support for kcontrols and DAPM widget controls.
Currently, it is limited to enumerated strings only.

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>