ARM: report Spectre v2 status through sysfs

commit 9dd78194a3722fa6712192cdd4f7032d45112a9a upstream.

As per other architectures, add support for reporting the Spectre
vulnerability status via sysfs CPU.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
[ preserve res variable - gregkh ]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>