target-arm: Add mp-affinity property for ARM CPU class

This allows to override default affinity IDs on a per-machine basis, and
possibility to retrieve IDs will be used by vGICv3 live migration code.

Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Shannon Zhao <shannon.zhao@linaro.org>
Tested-by: Shannon Zhao <shannon.zhao@linaro.org>
Message-id: 1465915112-29272-6-git-send-email-peter.maydell@linaro.org
