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/common/cmd_pci.c b/common/cmd_pci.c
index e3a77e3..dcecef8 100644
--- a/common/cmd_pci.c
+++ b/common/cmd_pci.c
@@ -48,6 +48,7 @@
 	unsigned char HeaderType;
 	unsigned short VendorID;
 	pci_dev_t dev;
+	int ret;
 
 	if (!hose)
 		return;
@@ -74,7 +75,10 @@
 			if (pci_skip_dev(hose, dev))
 				continue;
 
-			pci_read_config_word(dev, PCI_VENDOR_ID, &VendorID);
+			ret = pci_read_config_word(dev, PCI_VENDOR_ID,
+						   &VendorID);
+			if (ret)
+				goto error;
 			if ((VendorID == 0xFFFF) || (VendorID == 0x0000))
 				continue;
 
@@ -91,8 +95,12 @@
 				       BusNum, Device, Function);
 				pci_header_show(dev);
 			}
-	    }
-    }
+		}
+	}
+
+	return;
+error:
+	printf("Cannot read bus configuration: %d\n", ret);
 }