cxl/pmem: Add support for multiple nvdimm-bridge objects

cxl/pmem: Add support for multiple nvdimm-bridge objects

In preparation for a mocked unit test environment for CXL objects, allow
for multiple unique nvdimm-bridge objects.

For now, just allow multiple bridges to be registered. Later, when there
are multiple present, further updates are needed to
cxl_find_nvdimm_bridge() to identify which bridge is associated with
which CXL hierarchy for nvdimm registration.

Acked-by: Ben Widawsky <ben.widawsky@intel.com>
Link: https://lore.kernel.org/r/162982124325.1124374.4356765162960141442.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>