ARM: spectre: add Kconfig symbol for CPUs vulnerable to Spectre

Commit c58d237d0852a57fde9bc2c310972e8f4e3d155d upstream.

Add a Kconfig symbol for CPUs which are vulnerable to the Spectre
attacks.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Boot-tested-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: David A. Long <dave.long@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>