ASoC: SOF: add a field to store the current D0 substate of DSP

Add field d0_substate to struct snd_sof_dev to store the current DSP
D0 sub-state(only meaningful when DSP in D0), which could be D0I0 or
D0I3.

Signed-off-by: Keyon Jie <yang.jie@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20191025224122.7718-2-pierre-louis.bossart@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>