Support for Suspend

[qzed]

Suspend doesn't seem to work properly. Part of it may be #26, but there may be other remaining issues. This should only be an overall tracking issues, specific problems should be handled in their own issue. The device should at least support S2Idle, hopefully we can also support suspend to ram.

[qzed]

Suspend seems to work (mostly?) fine when I boot the system fully to ram and disable PCIe in the device tree. There are a couple of USB related errors/warnings, so that might also cause problems (could be "only" higher power draw when suspended).

In essence, this looks like PCIe is the main culprit, causing the observed lock-ups and device reboots (maybe caused by an SError).

[qzed]

Adapting steev/linux@9ec798b for sc8180x prevents the PCIe suspend problems. There are still some pciehp timeouts and USB PHY power-state warnings as well as an xHC error. Also, the system takes quite a bit to resume. So there are still some kinks to work out.