Restrict when we install arm64 spectre-v2 errata

[andrew]

Summary

When an arm64 CPU implements the CSV2 feature we don't need to install
the branch prediction hardening handler. This is because training a
branch predictor in one hardware context can't control speculative
execution in another context.

Sponsored by: Arm Ltd

[emaste]

Does this start applying the workaround to older thunderx?

[andrew]

It will apply the fix if it has the SMCCC handler to workaround the issue implemented in firmware.