arm64: add sysfs vulnerability show for meltdown

[ Upstream commit 1b3ccf4be0e7be8c4bd8522066b6cbc92591e912 ]

We implement page table isolation as a mitigation for meltdown.
Report this to userspace via sysfs.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@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>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>