ASoC: SOF: sort out Kconfig, again

ASoC: SOF: sort out Kconfig, again

The imx8 config keeps causing issues:

WARNING: unmet direct dependencies detected for SND_SOC_SOF_IMX8M
  Depends on [n]: SOUND [=y] && !UML && SND [=y] && SND_SOC [=m] && SND_SOC_SOF_TOPLEVEL [=y] && SND_SOC_SOF_IMX_TOPLEVEL [=y] && IMX_DSP [=n]
  Selected by [m]:
  - SND_SOC_SOF_IMX_OF [=m] && SOUND [=y] && !UML && SND [=y] && SND_SOC [=m] && SND_SOC_SOF_TOPLEVEL [=y] && SND_SOC_SOF_IMX_TOPLEVEL [=y] && SND_SOC_SOF_IMX8M_SUPPORT [=y]

This is complicated by two drivers having dependencies on both
platform specific drivers and the SND_SOC_SOF_OF framework code,
and using an somewhat obscure method to build them the same way
as the SOC_SOF_OF symbol (built-in or modular).

My solution now ensures that the two drivers can only be enabled
when the dependencies are met:

- When the platform specific drivers are built-in, everything is
  fine, as SOC_SOF_OF is either =y or =m

- When both are loadable modules, it also works, both for Kconfig
  and at runtime

- When the hardware drivers are loadable modules or disabled, and
  SOC_SOF_OF=y, prevent the IMX_SOF_OF drivers from being turned on,
  as this would be broken.

It seems that this is just an elaborate way to describe two tristate
symbols that have straight dependencies, but maybe I'm missing some
subtle point. It seems to always build for me now.

Fixes: fe57a92c8858 ("ASoC: SOF: Add missing dependency on IMX_SCU")
Fixes: afb93d716533 ("ASoC: SOF: imx: Add i.MX8M HW support")
Fixes: cb0312f61c3e ("ASoC: SOF: imx: fix undefined reference issue")
Fixes: afb93d716533dd ("ASoC: SOF: imx: Add i.MX8M HW support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Acked-by: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://lore.kernel.org/r/20200428212752.2901778-1-arnd@arndb.de
Signed-off-by: Mark Brown <broonie@kernel.org>