update spectre v2 microcodes blacklist

[Alexander Sergeyev]

Hello,

Intel released an updated microcode revision guidance [1] (8 Feb). The link is 
available in press-release [2], but it's quite easy to miss.

Some microcode revisions that were blacklisted before are now tested and 
pronounced production-ready:

> Lines with â***â were previously recommended to discontinue use. 
> Subsequent testing by Intel has determined that these were unaffected 
> by the stability issues and have been re-released without 
> modification.

I didn't fully match the updated revision guidance and spectre_bad_microcodes[] 
in arch/x86/kernel/cpu/intel.c, but at least microcode 0xc2 for intel skylakes 
should not be blacklisted anymore.

The update is also revelant to a recent blacklist backport to 4.9-stable:
https://patchwork.kernel.org/patch/10209021/

[1] https://newsroom.intel.com/wp-content/uploads/sites/11/2018/02/microcode-update-guidance.pdf
[2] https://newsroom.intel.com/news/security-issue-update-progress-continues-firmware-updates/