ASoC: SOF: ipc: Introduce SOF_IPC_GLB_TEST_MSG IPC command

Add a new class of IPC command along with the first
test type, IPC_FLOOD, which will be used for flooding the DSP
with IPCs.

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>