Avoid calling cpu_pm functions for retention idle states

CPU_PM_CPU_IDLE_ENTER() treats all idle states whose idx != 0 as a state that loses some context, but we can have deeper idle states that doesn't lose any software context. If a CPU is entering such a low power idle state where it retains the context, then there is no need to call cpu_pm_enter()/cpu_pm_exit().

Add a new macro(CPU_PM_CPU_IDLE_ENTER_RETENTION) to be used by cpuidle drivers when they are entering retention state. By not calling cpu_pm_enter and cpu_pm_exit we reduce the latency involved in entering and exiting the retention states.

On ARM64 based Qualcomm server platform we measured below overhead for for calling cpu_pm_enter and cpu_pm_exit for retention states.

workload: stress --hdd #CPUs --hdd-bytes 32M -t 30 Overhead of cpu_pm_enter - 1.2us(Average), 6.5us(Max) Overhead of cpu_pm_exit - 3.1us(Average), 11.1us(Max)

Listed below are 5 functions that were notified on ENTER/EXIT on the test platform:
gic_cpu_pm_notifier
arch_timer_cpu_pm_notify
cpu_pm_pmu_notify
hyp_init_cpu_pm_notifier
fpsimd_cpu_pm_notifier

Prashanth Prakash (2):
cpuidle: Add new macro to enter a retention idle state ARM64 / cpuidle: Use new cpuidle macro for entering retention state

Changes in v3:
- Added additional clarification to commit message (Sudeep)

Changes in v2:
- Reordered cpuidle.h macros for better readablity (Rafael)

 arch/arm64/kernel/cpuidle.c |  8 +++++++-
 include/linux/cpuidle.h     | 40 ++++++++++++++++++++++++----------------
 2 files changed, 31 insertions(+), 17 deletions(-)

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