ASoC: Intel: rename SND_SST_MFLD_PLATFORM to SND_SST_ATOM_HIFI2_PLATFORM

Rename SND_SST_MFLD_PLATFORM to SND_SST_ATOM_HIFI2_PLATFORM to make it clear
that is not only about Medfield platform.

The new name is derived from Intel Atom and HiFi2. HiFi2 is the DSP version,
it's public information for Intel *Field/*Trail parts, see
https://www.alsa-project.org/main/index.php/Firmware

. By combining HiFi2 with
Atom we get a unique non-ambiguous description of the core+DSP hardware for
Intel Medfield through Intel Cherrytrail.

Suggested-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Acked-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>