cxl/core: Refactor CXL register lookup for bridge reuse

cxl/core: Refactor CXL register lookup for bridge reuse

While CXL Memory Device endpoints locate the CXL MMIO registers in a PCI
BAR, CXL root bridges have their MMIO base address described by platform
firmware. Refactor the existing register lookup into a generic facility
for endpoints and bridges to share.

Reviewed-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/162096972534.1865304.3218686216153688039.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>