ARM: cpuidle: Register per cpuidle device

If the cpuidle init cpu operation returns -ENXIO, therefore reporting HW
failure or misconfiguration, the CPUidle driver skips the respective
cpuidle device initialization because the associated platform back-end HW
is not operational.

That prevents the system to crash and allows to handle the error gracefully.

For example, on Qcom's platform, each core has a SPM. The device associated
with this SPM is initialized before the cpuidle framework. If there is an error
in the initialization (eg. error in the DT), the system continues to boot but
in degraded mode as some SPM may not be correctly initialized.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>