cxl/test: Setup target_map for cxl_test decoder initialization

cxl/test: Setup target_map for cxl_test decoder initialization

cxl_test uses mock functions for decoder enumaration. Add initialization
of the cxld->target_map[] for cxl_test based decoders in the mock
functions.

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Tested-by: Robert Richter <rrichter@amd.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>