The additional-devs subnode allows to declared some virtual devices
as sound card children.
These virtual devices can then be used by the sound card and so be
present in the audio path.

The first virtual device supported is the audio IIO auxiliary device
in order to support an IIO device as an audio auxiliary device.
Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Christophe Leroy <christophe.leroy@©sgroup.eu>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/r/20230623085830.749991-3-herve.codina@bootlin.comSigned-off-by: Mark Brown <broonie@kernel.org>