ASoC: SOF: move definition of snd_sof_ipc to header file

Move definition of struct snd_sof_ipc to the header file so it can be
shared with new IPC versions.

Signed-off-by: Rander Wang <rander.wang@intel.com>
Reviewed-by: Péter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Link: https://lore.kernel.org/r/20220308164344.577647-19-ranjani.sridharan@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>