arm64: Add sysfs vulnerability show for spectre-v1

spectre-v1 has been mitigated and the mitigation is always active.
Report this to userspace via sysfs

Signed-off-by: Mian Yousaf Kaukab <ykaukab@suse.de>
Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>