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: I10e3ca40888379b8f56c21f0ddf488a4002f4a31
Co-developed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Patch-mainline: linux-arm-kernel @ 01/08/18, 9:32
[ckadabi@codeaurora.org: Resolved minor conflcits]
Signed-off-by: Channagoud Kadabi <ckadabi@codeaurora.org>