ASoC: wm_adsp: Expose mixer control API

Expose mixer control API for reading and writing controls from the kernel.
This API can be used by ALSA kernel drivers with ADSP support to
read and write firmware-defined memory regions.

Signed-off-by: Li Xu <li.xu@cirrus.com>
Signed-off-by: David Rhodes <david.rhodes@cirrus.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/1573847653-17094-2-git-send-email-david.rhodes@cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>