Slowly but surely, the Seattle topic branch is coming back to life.
I pushed up a version today to acpi.git that has the console working (and earlycon) via SPCR, AHCI works so the SATA drive will come up, and the PMU will start, and the secondary CPUs will come up via the parking protocol. GICv2 support seems to be working. too. I recommend that you have version 73C or 74D firmware; anything earlier is sketchy.
This is all in acpi-topic-seattle, which is based on the v9 version of the patches (and hence 4.0-rc1, too). It's enough I can get to a shell prompt/login and do useful things.
Things that still need work:
-- Reboot doesn't work; there's a patch for this but not applied just yet
-- The XGBE NIC is weird. If you have the B0 rev of the board, it may work fine. I have an A0 which requires a slightly different driver that is not working (it causes a hang that I'm trying to figure out); this may just be because of 4.0.
-- Need to add in PCI support, plus things like USB, I2C, IPMI, and so on -- all the other stuff currently listed in the DT. There are patches for all of this, but it's a mess because of what we can and cannot release just yet -- and, well, because I find PCI confusing, too :).
-- KVM would be good to have (yes, it does need a mod for ACPI).
I'd like to at least get PCI working before notifying LKML.
Anything you have time to help with is greatly appreciated....