cxl/core: Replace unions with struct_group()

cxl/core: Replace unions with struct_group()

Use the newly introduced struct_group_typed() macro to clean up the
declaration of struct cxl_regs.

Cc: Alison Schofield <alison.schofield@intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Ben Widawsky <ben.widawsky@intel.com>
Cc: linux-cxl@vger.kernel.org
Suggested-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/lkml/1d9a2e6df2a9a35b2cdd50a9a68cac5991e7e5f0.camel@intel.com
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Kees Cook <keescook@chromium.org>