arm: Clean up psci_cpu_stopped usage

psci_cpu_stopped returns a bool, so let's use it like this.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
arm: Wait for CPU to stop in arch_suspend_cpu

The semantic of arch_suspend_cpu is synchronous, i.e. it has to wait
until the target CPU was actually suspended. Extend the ARM
implementation accordingly.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>