stable backport request, add cortex-a cpus to whitelist

Hello,

I would like to request 2a355ec25729 ("arm64: kpti: Whitelist Cortex-A CPUs that don't implement the CSV3 field")

to be backported to 4.19 stable.

These CPUs are not susceptible to Meltdown, so enabling the mitigations for Meltdown (kpti) should be redundant, especially since we know that it can have a huge performance penalty for certain workloads.

kpti will still be automatically enabled if KASLR is enabled.

Kind regards, Niklas