cpuidle: cpuidle-apple: Reduce target residency for CPU Power Down state - #532

Can you explain what is the purpose of this change and what exactly it does?

Summary of the change

This patch reduces the target_residency of the "CPU PD" (Power Down) state from 10,000µs (10ms) to 1,500µs (1.5ms) to allow the CPU/cluster to enter the deeper idle state more aggressively during short idle windows.

Technical Justification

1. Overestimated Residency: The current value of 10,000µs is excessively high for a modern core architecture like Apple Silicon. Given that the exit_latency is only 10µs, the energy break-even point (the minimum time required to offset the power cost of entering and exiting the power-down state) is much lower than 10ms.

2. Energy Efficiency in Shallow Idle Patches:
   With a 10ms target residency, the kernel's cpuidle governor (menu or teo) frequently falls back to shallower idle states (or fails to power down the cluster) during typical bursty workloads, even when the CPU could have safely powered down and saved power.

3. Alignment with Practical Latency:
   Lowering the threshold to 1,500µs ensures a better balance. It allows the cpuidle governor to select the "CPU PD" state more dynamically during short intervals, significantly improving battery life and thermal efficiency without introducing regression in latency overhead.

I think this PR is at a borderline...it can be considered as an enhancement but when seeing this from different perspective this is a ricing