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>
diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
index e69de29..8b7e2ee 100644
--- a/drivers/pci/Kconfig
+++ b/drivers/pci/Kconfig
@@ -0,0 +1,12 @@
+menu "PCI"
+
+config DM_PCI
+	bool "Enable driver mode for PCI"
+	depends on DM
+	help
+	  Use driver model for PCI. Driver model is the new method for
+	  orgnising devices in U-Boot. For PCI, driver model keeps track of
+	  available PCI devices, allows scanning of PCI buses and provides
+	  device configuration support.
+
+endmenu