arm64: Take into account ID_AA64PFR0_EL1.CSV3

commit 179a56f6f9fb upstream.

For non-KASLR kernels where the KPTI behaviour has not been overridden
on the command line we can use ID_AA64PFR0_EL1.CSV3 to determine whether
or not we should unmap the kernel whilst running at EL0.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
[Alex: s/read_sanitised_ftr_reg/read_system_reg/ to match v4.9 naming]
Signed-off-by: Alex Shi <alex.shi@linaro.org> [v4.9 backport]
[Mark: correct zero bits in ftr_id_aa64pfr0 to account for CSV3]
Signed-off-by: Mark Rutland <mark.rutland@arm.com> [v4.9 backport]
Tested-by: Will Deacon <will.deacon@arm.com>
Tested-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>