ASoC: SOF: debug: add new debugfs entries for IPC flood test

Add a couple of new debugfs entries "ipc_flood_count"
and "ipc_flood_duration_ms" that can be used to
execute the IPC flood test. "ipc_flood_count" floods the DSP
with the number of test IPCs specified and ipc_flood_duration_ms
floods the DSP with test IPC's for the duration(in ms) specified.
The test stats such as average, min and max IPC response times
are logged in the dmesg and saved in the debugfs entry cache buffer.

Signed-off-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>