dm: pci: Add a uclass for PCI

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>