logging: allow to select FORMAT_TIMESTAMP for ADSP_MTRACE backend

The CONFIG_LOG_BACKEND_FORMAT_TIMESTAMP cannot be selected unless one of
the enumerated backends is selected. Add the ADSP_MTRACE backend to the
list, so timestamp formatting can be selected when only this backend is
enabled in the build.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>