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: fe57a92c ("ASoC: SOF: Add missing dependency on IMX_SCU")
Fixes: afb93d71 ("ASoC: SOF: imx: Add i.MX8M HW support")
Fixes: cb0312f6 ("ASoC: SOF: imx: fix undefined reference issue")
Fixes: afb93d71 ("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.deSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sof/imx/Kconfig

@@ -21,7 +21,8 @@ config SND_SOC_SOF_IMX_OF
 
 config SND_SOC_SOF_IMX8_SUPPORT
 	bool "SOF support for i.MX8"
-	depends on IMX_SCU
+	depends on IMX_SCU=y || IMX_SCU=SND_SOC_SOF_IMX_OF
+	depends on IMX_DSP=y || IMX_DSP=SND_SOC_SOF_IMX_OF
 	help
 	  This adds support for Sound Open Firmware for NXP i.MX8 platforms
 	  Say Y if you have such a device.
@@ -29,14 +30,13 @@ config SND_SOC_SOF_IMX8_SUPPORT
 
 config SND_SOC_SOF_IMX8
 	tristate
-	depends on IMX_SCU
-	select IMX_DSP
 	help
 	  This option is not user-selectable but automagically handled by
 	  'select' statements at a higher level
 
 config SND_SOC_SOF_IMX8M_SUPPORT
 	bool "SOF support for i.MX8M"
+	depends on IMX_DSP=y || IMX_DSP=SND_SOC_SOF_OF
 	help
 	  This adds support for Sound Open Firmware for NXP i.MX8M platforms
 	  Say Y if you have such a device.
@@ -44,7 +44,6 @@ config SND_SOC_SOF_IMX8M_SUPPORT
 
 config SND_SOC_SOF_IMX8M
 	tristate
-	depends on IMX_DSP
 	help
 	  This option is not user-selectable but automagically handled by
 	  'select' statements at a higher level