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

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

commit 0f15adbb2861 upstream.

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>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Alex Shi <alex.shi@linaro.org>
Conflicts:
	expand enable_da_f in entry.S
	use 5 parameters ARM64_FTR_BITS()
	add percpu.h in mm_types.h for percpu functions
	use cpus_have_cap instead of cpus_have_const_cap
	arch/arm64/Kconfig
	arch/arm64/include/asm/cpucaps.h
	arch/arm64/include/asm/mmu.h
	arch/arm64/include/asm/sysreg.h
	arch/arm64/kernel/cpufeature.c
	arch/arm64/kernel/entry.S
	arch/arm64/mm/fault.c