arm64: Add per-cpu infrastructure to call ARCH_WORKAROUND_2

In a heterogeneous system, we can end up with both affected and
unaffected CPUs. Let's check their status before calling into the
firmware.

Change-Id: Ie938e9c0ec0ae85e070c3a80d9b6043208471edb
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Git-commit: 5cf9ce6e5ea50f805c6188c04ed0daaec7b6887d
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[isaacm@codeaurora.org: define per cpu var only when ssbd
 config option is set.]
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>