cxl/core: Move target_list out of base decoder attributes

In preparation for introducing endpoint decoder objects, move the
target_list attribute out of the common set since it has no meaning for
endpoint decoders.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Ben Widawsky <ben.widawsky@intel.com>
Link: https://lore.kernel.org/r/164298430100.3018233.4715072508880290970.stgit@dwillia2-desk3.amr.corp.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>