Implement s2idle low power state

[alice-sowerby]

User Story

As a laptop user I want to be able to save battery while not actively working so that the laptop can be used longer between charging.

Implementation Notes

Forward-porting and patching the s2idle work that was done . Put into review/test with Kostik, John B... Aim to have this existing patch into the tree by end Dec.

We have a few goals with s2idle/s0ix:

s2idle -- not particularly useful by itself, but a dependency of further work
as a replacement for no-longer-available S3 -- "traditional" suspend, where the user explicitly stops working (closes the lid, types zzz etc.) and the laptop resumes later on
to support better power management while in use (e.g. while a user is just reading a web page much of the computer is idle, and unused components could be put into lower power states)

Notes on S2Idle
Source
State: Suspend-To-Idle
ACPI state: S0 (shallowest of 4 levels)
Label: "s2idle" ("freeze")

This state is a generic, pure software, light-weight, system sleep state.
It allows more energy to be saved relative to runtime idle by freezing user space and putting all I/O devices into low-power states (possibly lower-power than available at run time), such that the processors can
spend more time in their idle states.

Acceptance Criteria

• [ ]

[emaste]

We have a few goals with s2idle/s0ix:

1. s2idle -- not particularly useful by itself, but a dependency of further work
2. as a replacement for no-longer-available S3 -- "traditional" suspend, where the user explicitly stops working (closes the lid, types zzz etc.) and the laptop resumes later on
3. to support better power management while in use (e.g. while a user is just reading a web page much of the computer is idle, and unused components could be put into lower power states)