plat: Hikey960: fix the CPU hotplug

In CPU off callback function, the old code uses the function
hisi_test_pwrdn_allcores() to check if all CPUs in cluster have been
powered off and if it's valid then power off the whole cluster. But the
function hisi_test_pwrdn_allcores() only maintains the different power
states only for CPU suspend/resume flow, so it cannot return correct
states for CPU on/off flow.

This patch is to change use hisi_test_cpu_down() to check if all CPUs
have been powered off, so that can power off the whole cluster properly
when all CPUs in cluster have been hotplugged off.

Signed-off-by: Tao Wang <kevin.wangtao@hisilicon.com>
Signed-off-by: Leo Yan <leo.yan@linaro.org>