Add a uclass for PCI controllers and a generic one for PCI devices. Adjust
the 'pci' command and the existing PCI support to work with this new uclass.
Keep most of the compatibility code in a separate file so that it can be
removed one day.
TODO: Add more header file comments to the new parts of pci.h
Signed-off-by: Simon Glass <sjg@chromium.org>
