intel_adsp: Rename multiprocessing cavs file

For intel adsp platforms we have a common multiprocessing.c file and
then another multiprocessing.c for ace and multiprocessing_cavs.c for
cavs. Rename the cavs specific implementation to follow the convention.
There is not need to "cavs" suffix since the file is already inside its
specific directory.

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