x86: pci: Allow configuration before relocation

Add simple PCI access routines for x86 which permit use before relocation.
The normal PCI stack is still used, but for pre-relocation use there can
only ever be a single hose. After relocation, fall back to the normal
access, although even then on x86 machines there is normally only a single
PCI bus.

Signed-off-by: Simon Glass <sjg@chromium.org>