Hikey960: fix PSCI suspend stuck issue

Hikey960: fix PSCI suspend stuck issue

Clear the cpuidle flag when resuming from idle. This flag is set
when entering idle, and if it remains set when resuming, it can
prevent the cluster from powering off during the next system
suspend operation. During system suspend, all CPUs are plugged
out except the last CPU, which is suspended. If any of the
cpuidle flags are set at this point, the last CPU will be stuck
in a WFI loop and will not be powered off.
This problem only occurs during system suspend.

Signed-off-by: Tao Wang <kevin.wangtao@linaro.org>