efi/cper, cxl: Remove cxl_err.h

While going to create include/linux/cxl.h for some cross-subsystem CXL
definitions I noticed that include/linux/cxl_err.h was already present.
That header has no reason to be global, and it duplicates the RAS
Capability Structure definitions in drivers/cxl/cxl.h. A follow-on patch
can consider unifying the CXL native error tracing with the CPER error
printing.

Also fixed up the spec reference as the latest released spec is v3.0.

Cc: Smita Koralahalli <Smita.KoralahalliChannabasappa@amd.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>