cxl: Flesh out register names

Get a better naming scheme in place for upcoming additions. By dropping
redundant usages of CXL and DVSEC where appropriate we can get more
concise and also more grepable defines.

Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
Link: https://lore.kernel.org/r/164298414022.3018233.15522855498759815097.stgit@dwillia2-desk3.amr.corp.intel.com


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