ARM: cpuidle: silence error on driver registration failure

It's perfectly fine to have multiple cpuidle driver compiled in the
build configuration. However, it's not good to throw error on driver
registration failure if some other driver is already initialised and
assigned. In such cases, __cpuidle_register_driver returns -EBUSY and
we can check for such error before throwing the error.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>