[coreboot] subsystem IDs

[Myles Watson]

This patch makes subsystem ids work.  Here are the changes by file:

device/pci_device.c:
    Only update IDs if:
    - The device is on the mainboard
    - The device has a Vendor ID and Device ID
    - The device has a set_subsystem function in ops_pci(dev)

util/dtc/flattree.c:
    Make devices from the dts be on_mainboard.
    If they're plugged in, they shouldn't be in the dts.

mainboard/amd/serengeti/dts:
    Add subsystem_vendor and subsystem_device.

Build tested on Serengeti.  Getting closer :)

Signed-off-by: Myles Watson

Thanks,
Myles