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.

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>

arch/arm64/kernel/cpu_errata.c

@@ -233,6 +233,8 @@ enable_smccc_arch_workaround_1(const struct arm64_cpu_capabilities *entry)
 #endif	/* CONFIG_HARDEN_BRANCH_PREDICTOR */
 
 #ifdef CONFIG_ARM64_SSBD
+DEFINE_PER_CPU_READ_MOSTLY(u64, arm64_ssbd_callback_required);
+
 void __init arm64_update_smccc_conduit(struct alt_instr *alt,
 				       __le32 *origptr, __le32 *updptr,
 				       int nr_inst)