intel_adsp: Move cavs common files to a subdir

Inside the common directory there were files that are CAVS specific and
are not used by ACE_V1X. Lets create a subdir called cavs inside the
common to put files that are common for only cavs plaftorms.

Note that there are still remaining code that in the common folder that
are using cavs namespace like "cavs_ipc_*" that may need some additional
work.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>