ASoC: SOF: sof-audio: Update documentation for sof_ipc_tplg_ops

ASoC: SOF: sof-audio: Update documentation for sof_ipc_tplg_ops

The core treats all function pointer in sof_ipc_tplg_ops as optional.
Update the documentation to reflect this.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Reviewed-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Reviewed-by: Rander Wang <rander.wang@intel.com>
Link: https://lore.kernel.org/r/20221221102328.9635-3-peter.ujfalusi@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>