ASoC: SOF: Intel: Baytrail: clarify mutual exclusion with Atom/SST driver

Some distros select all options blindly, which leads to confusion and
bug reports. Since SOF does not support Baytrail-CR for now, and
UCM/topology files are still being propagated to downstream distros,
make SOF on Baytrail an opt-in option that first require distros to
opt-out of existing defaults.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20191101173045.27099-2-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>