ASoC: SOF: Kconfig: add EXPERT dependency for developer options, clarify help

ASoC: SOF: Kconfig: add EXPERT dependency for developer options, clarify help

Some distros select all possible options, despite existing warnings to
be careful. This leads to e.g. user reports that the HDaudio codec and
DMIC are not handled by SOF.

Add an explicit menu item to unlock developer options, and make them
dependent on CONFIG_EXPERT. Hopefully with this double-lock these
options will only be selected by developers.

GitHub issue: https://github.com/thesofproject/sof/issues/1885
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20191101173045.27099-5-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>