arm64: Implement branch predictor hardening for affected Cortex-A CPUs

Cortex-A57, A72, A73 and A75 are susceptible to branch predictor aliasing
and can theoretically be attacked by malicious code.

This patch implements a PSCI-based mitigation for these CPUs when available.
The call into firmware will invalidate the branch predictor state, preventing
any malicious entries from affecting other victim contexts.

Change-Id: I08acb4a1436709aee7e57ad5e14b37f1a8738038
Co-developed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Git-commit: aa6acde65e03186b5add8151e1ffe36c3c62639b
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux


[neeraju@codeaurora.org: resolve merge conflicts.
 Implement PSCI-based mitigation only for Cortex-A57]
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>