ARM: cpuidle: Support asymmetric idle definition

Some hardware have clusters with different idle states. The current code does
not support this and fails as it expects all the idle states to be identical.

Because of this, the Mediatek mtk8173 had to create the same idle state for a
big.Little system and now the Hisilicon 960 is facing the same situation.

Solve this by simply assuming the multiple driver will be needed for all the
platforms using the ARM generic cpuidle driver which makes sense because of the
different topologies we can support with a single kernel for ARM32 or ARM64.

Every CPU has its own driver, so every single CPU can specify in the DT the
idle states.

This simple approach allows to support the future dynamIQ system, current SMP
and HMP.

Tested on:
 - 96boards: Hikey 620
 - 96boards: Hikey 960
 - 96boards: dragonboard410c
 - Mediatek 8173

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