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.

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>

arch/arm64/Kconfig

@@ -940,6 +940,15 @@ config HARDEN_EL2_VECTORS
 
 	  If unsure, say Y.
 
+config ARM64_SSBD
+	bool "Speculative Store Bypass Disable" if EXPERT
+	default y
+	help
+	  This enables mitigation of the bypassing of previous stores
+	  by speculative loads.
+
+	  If unsure, say Y.
+
 menuconfig ARMV8_DEPRECATED
 	bool "Emulate deprecated/obsolete ARMv8 instructions"
 	depends on COMPAT