arm64: Add ARCH_WORKAROUND_2 probing

As for Spectre variant-2, we rely on SMCCC 1.1 to provide the
discovery mechanism for detecting the SSBD mitigation.

A new capability is also allocated for that purpose, and a
config option.

Change-Id: Ibf7a217bb6ecf796e50f79c2681de094f1caaa8e
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Git-commit: a725e3dda1813ed306734823ac4c65ca04e38500
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[isaacm@codeaurora.org: Move inclusion of psci header and
 arm-smccc header to beginning of source to not break
 compilation, and use correct capability numbers and
 structure fields. Also, disable SSBD mitigation for
 the time being.]
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>