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

Aliasing attacks against CPU branch predictors can allow an attacker 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.

Co-developed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
[ckadabi@codeaurora.org: Add changes to include header files for per cpu
declarations in mmu.h and use raw_cpu_ptr instead of this_cpu_ptr.
Disable branch predictor hardening by default.]
Patch-mainline: linux-arm-kernel @ 01/05/18, 5:12
Change-Id: Ic8cafac0db223ba3cfe7b55419b29cea7b297a72
Signed-off-by: Channagoud Kadabi <ckadabi@codeaurora.org>