ASoC: SOF: imx: add missing MODULE_LICENSE() for imx-common

ASoC: SOF: imx: add missing MODULE_LICENSE() for imx-common

Fix build warning:
WARNING: modpost: missing MODULE_LICENSE() in sound/soc/sof/imx/imx-common.o

Fixes: 18ebffe4d043 ("ASoC: SOF: imx: Add debug support for imx platforms")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Reviewed-by: Daniel Baluta <daniel.baluta@gmail.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200921104746.2903507-1-kai.vehmanen@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>