Implement Spectre-V4 mitigations. The only real effect of this change is

Implement Spectre-V4 mitigations.  The only real effect of this change is
that we now make a firmware call to enable the mitigations if the
firmware tells us mitigations are implemented and needed.  But according
to the specification these mitigations should be enabled by default.
The open source TF-A implementation only implements mitigations for older
Cortex-A76 cores.  Newer Cortex-A76 revisions are not vulnerable and as
far as I can tell we only support SoCs with the newer cores.

ok patrick@