x86/cpu: Comment Skylake server stepping too

Further to

  53375a5a ("x86/cpu: Resort and comment Intel models"),

CascadeLake and CooperLake are steppings of Skylake, and make up the 1st
to 3rd generation "Xeon Scalable Processor" line.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210409121027.16437-1-andrew.cooper3@citrix.com