KVM: arm/arm64: Documentation: Update arm-vgic-v3.txt

Update error code returned for Invalid CPU interface register
value and access in AArch32 mode.

Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Vijaya Kumar K <Vijaya.Kumar@cavium.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>