Prevent AMD laptops from waking sporadically when put to sleep with S0ix

[obiwac]

User Story

As an AMD laptop user I want to be able to put my laptop to sleep using S0ix without it waking sporadically so that I can extend the battery life of my laptop.

Implementation Notes

Currently, suspending to idle on AMD laptops which use S0ix (#32) sporadically wakes because the EC often interrupts the CPU for non-wake events. These events can't be masked as the EC is also responsible for other important wake events such as the lid opening or the power button being pressed which we definitely do want to interrupt the idling CPU. Since they're all emitted by the EC they all share the same GPE number.

I believe an AMD PMC driver, which can be used to tell the SMU (system management unit, a power management chip on AMD CPUs) when the system intends to enter a deep lower power idle state, will fix this problem. See amd-pmc on Linux.

Acceptance Criteria

• AMD laptops can be put to sleep normally without sporadically waking.

[obiwac]

I have started working on an AMD PMC driver: obiwac/freebsd-s0ix#5