tests: Define mbox device

XCC (based on GCC 4.2) will emit undefined references to symbols that
are not used. Newer versions of GCC simply don't emit them, but for XCC,
linker will complain about those undefined references.

This patch adds a dummy definition of the mbox devices used on tests, so
that the linker is happy.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>