arm64: Add skeleton to harden the branch predictor against aliasing attacks

Aliasing attacks against CPU branch predictors can allow the attacks to
redirect speculative control flow on some CPUs and potentially divulge
information from one context to another.

This patch adds initial skeleton code behind a new Kconfig option to
enable implementation-specific mitigations against these attacks for
CPUs that are affected.

Change-Id: I07fba1943dd63df8951bf68fac947666100e5559
Co-developed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Git-commit: 0f15adbb2861ce6f75ccfc5a92b19eae0ef327d0
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[sramana@codeaurora.org: Fix merge conflicts and make it
compilable on msm-4.4]
Signed-off-by: Srinivas Ramana <sramana@codeaurora.org>