The function arm_enter_idle_state is exactly the same in both generic
ARM{32,64} CPUIdle driver and will be the same even on ARM64 backend
for ACPI processor idle driver. So we can unify it and move it to a
common place by introducing CPU_PM_CPU_IDLE_ENTER macro that can be
used in all places avoiding duplication.

This is in preparation of reuse of the generic cpuidle entry function
for ACPI LPI support on ARM64.
Suggested-by: Rafael J. Wysocki <rjw@rjwysocki.net>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>