ASoC: SOF: OF: Avoid reverse module dependency

Similar with commit 8a49cd11 ("ASoC: SOF: ACPI: avoid reverse
module dependency") we will be having hardware specific drivers that
link against a common "helper" framework.

sof-of-dev.c becomes a library with the interface defined in the newly
created file sof-of-dev.h.

This is the final step started with Kconfig simplification in
commit 7548a391

 ("ASoC: SOF: i.MX: simplify Kconfig")

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Paul Olaru <paul.olaru@oss.nxp.com>
Reviewed-by: Péter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20211215085703.137414-1-daniel.baluta@oss.nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>