ASoC: Improve coverage in default KUnit runs

Merge series from Mark Brown <broonie@kernel.org>:

We have some KUnit tests for ASoC but they're not being run as much as
they should be since ASoC isn't enabled in the configs used by default
with KUnit and in the case of the topology tests there is no way to
enable them without enabling drivers that use them.  This series
provides a Kconfig option which KUnit can use directly rather than worry
about drivers.

Further, since KUnit is typically run in UML but ALSA prevents build
with UML we need to remove that Kconfig conflict.  As far as I can tell
the motiviation for this is that many ALSA drivers use iomem APIs which
are not available under UML and it's more trouble than it's worth to go
through and add per driver dependencies.  In order to avoid these issues
we also provide stubs for these APIs so there are no build time issues
if a driver relies on iomem but does not depend on it.  With these stubs
I am able to build all the sound drivers available in a UML defconfig
(UML allmodconfig appears to have substantial other issues in a quick
test).

With this series I am able to run the topology KUnit tests as part of a
kunit --alltests run.