arm64: kpti: Whitelist Cortex-A CPUs that don't implement the CSV3 field

commit 2a355ec2 upstream.

While the CSV3 field of the ID_AA64_PFR0 CPU ID register can be checked
to see if a CPU is susceptible to Meltdown and therefore requires kpti
to be enabled, existing CPUs do not implement this field.

We therefore whitelist all unaffected Cortex-A CPUs that do not implement
the CSV3 field.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Cc: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Conflicts:
  arch/arm64/kernel/cpufeature.c
[yyl: adjust context]

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>