ASoC: SOF: Add support for IPC IO between DSP and Host

Define an IPC ABI for all host <--> DSP communication. This ABI should
be transport agnostic. i.e. it should work on MMIO and SPI/I2C style
interfaces.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Signed-off-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@kernel.org>