cxl/pci: Convert register block identifiers to an enum

In preparation for passing around the Register Block Indicator (RBI) as
a parameter, it is desirable to convert the type to an enum so that the
interface can use a well defined parameter.

Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
[djbw: changelog fixups ]
Link: https://lore.kernel.org/r/163379784199.692348.4366131432595112771.stgit@dwillia2-desk3.amr.corp.intel.com


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