arm64: Force KPTI to be disabled on Cavium ThunderX

arm64: Force KPTI to be disabled on Cavium ThunderX

commit 6dc52b15c4a4 upstream.

Cavium ThunderX's erratum 27456 results in a corruption of icache
entries that are loaded from memory that is mapped as non-global
(i.e. ASID-tagged).

As KPTI is based on memory being mapped non-global, let's prevent
it from kicking in if this erratum is detected.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
[will: Update comment]
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:
	using old function read_system_reg/cpus_have_cap to replace
	read_sanitised_ftr_reg/cpus_have_const_cap in
	arch/arm64/kernel/cpufeature.c