x86: coreboot: Tell u-boot about PCI bus 0 when initializing

U-boot needs a host controller or "hose" to interact with the PCI busses
behind them. This change installs a host controller during initialization of
the coreboot "board" which implements some of X86's basic PCI semantics. This
relies on some existing generic code, but also duplicates a little bit of code
from the sc520 implementation. Ideally we'd eliminate that duplication at some
point.

It looks like in order to scan buses beyond bus 0, we'll need to tell u-boot's
generic PCI configuration code what to do if it encounters a bridge,
specifically to scan the bus on the other side of it.

Signed-off-by: Gabe Black <gabeblack@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Graeme Russ <graeme.russ@gmail.com>