tools/testing/cxl: Enumerate mock decoders

Enumerate 2-decoders per switch port and endpoint in the cxl_test
topology.

Link: https://lore.kernel.org/r/164298432699.3018233.12131068635065601541.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>