x86: coreboot: Implement recursively scanning PCI busses

x86: coreboot: Implement recursively scanning PCI busses

A hook is installed to configure PCI bus bridges as they encountered by u-boot.
The hook extracts the secondary bus number from the bridge's config space and
then recursively scans that bus.

On Coreboot, the PCI bus address space has identity mapping with the
physical address space, so declare it as such to ensure that the "pci_map_bar"
function used by some PCI drivers is behaving properly. This fixes the
EHCI PCI driver initialization on Stumpy.

This was tested as follows:

Ran the PCI command on Alex, saw devices on bus 0, the OXPCIe 952 on
bus 1, and empty busses 2 through 5. This matches the bridges
reported on bus 0 and the PCI configuration output from coreboot.

Signed-off-by: Gabe Black <gabeblack@chromium.org>
Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
Signed-off-by: Stefan Reinauer <reinauer@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>