ASoC: SOF: sof-pci-dev: add .shutdown() callback

Add the .shutdown() callback to the sof-pci-dev driver, to help to
handle shutting down specific tasks for SOF PCI platforms.

Signed-off-by: Keyon Jie <yang.jie@linux.intel.com>
Reviewed-by: Bard Liao <bard.liao@intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Link: https://lore.kernel.org/r/20210113152617.4048541-3-kai.vehmanen@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>