KVM: arm64: Enable GICv3 Group-0 sysreg trapping via command-line

Now that we're able to safely handle Group-0 sysreg access, let's
give the user the opportunity to enable it by passing a specific
command-line option (vgic_v3.group0_trap).

Tested-by: Alexander Graf <agraf@suse.de>
Acked-by: David Daney <david.daney@cavium.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@linaro.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -1829,6 +1829,10 @@
 			for all guests.
 			Default is 1 (enabled) if in 64-bit or 32-bit PAE mode.
 
+	kvm-arm.vgic_v3_group0_trap=
+			[KVM,ARM] Trap guest accesses to GICv3 group-0
+			system registers
+
 	kvm-arm.vgic_v3_group1_trap=
 			[KVM,ARM] Trap guest accesses to GICv3 group-1
 			system registers