cxl: Add a cached copy of target_map to cxl_decoder

cxl: Add a cached copy of target_map to cxl_decoder

Add a cached copy of the hardware port-id list that is available at init
before all @dport objects have been instantiated. Change is in preparation
of delayed dport instantiation.

Reviewed-by: Robert Richter <rrichter@amd.com>
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>