ASoC: Intel: boards: Fix CONFIG_SND_SOC_SDW_MOCKUP select

When CONFIG_SND_SOC_INTEL_SOUNDWIRE_SOF_MACH is enabled without
CONFIG_EXPERT, there is a Kconfig warning about unmet dependencies:

WARNING: unmet direct dependencies detected for SND_SOC_SDW_MOCKUP
  Depends on [n]: SOUND [=y] && !UML && SND [=y] && SND_SOC [=y] &&
EXPERT [=n] && SOUNDWIRE [=y]
  Selected by [y]:
  - SND_SOC_INTEL_SOUNDWIRE_SOF_MACH [=y] && ...

Selecting a symbol does not account for dependencies. There are three
ways to resolve this:

1. Make CONFIG_SND_SOC_INTEL_SOUNDWIRE_SOF_MACH select
   CONFIG_SND_SOC_SDW_MOCKUP only if CONFIG_EXPERT is set.

2. Make CONFIG_SND_SOC_SDW_MOCKUP's prompt depend on CONFIG_EXPERT so
   that it can be selected by options that only depend on
   CONFIG_SOUNDWIRE but still appear as a prompt to the user when
   CONFIG_EXPERT is set.

3. Make CONFIG_SND_SOC_INTEL_SOUNDWIRE_SOF_MACH imply
   CONFIG_SND_SOC_SDW_MOCKUP, which will select
   CONFIG_SND_SOC_SDW_MOCKUP when its dependencies are enabled but still
   allow the user to disable it.

Go with the third option as it gives the most flexibility while
retaining the original intent of the select.

Fixes: 0ccac3bc

 ("ASoC: Intel: boards: sof_sdw: add SoundWire mockup codecs for tests")
Suggested-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20210902181217.2958966-1-nathan@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>