hw/arm/virt: Add PMU node for virt machine

Add a virtual PMU device for virt machine while use PPI 7 for PMU
overflow interrupt number.

Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-id: 1465267577-1808-3-git-send-email-zhaoshenglong@huawei.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
