dm: usb: Add driver model support for hubs

Adjust the existing hub code to support driver model, and add a USB driver
for hubs.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>
diff --git a/include/dm/uclass-id.h b/include/dm/uclass-id.h
index 95bd249..967efec 100644
--- a/include/dm/uclass-id.h
+++ b/include/dm/uclass-id.h
@@ -41,6 +41,7 @@
 	UCLASS_ETH,		/* Ethernet device */
 	UCLASS_LPC,		/* x86 'low pin count' interface */
 	UCLASS_USB,		/* USB bus */
+	UCLASS_USB_HUB,		/* USB hub */
 
 	UCLASS_COUNT,
 	UCLASS_INVALID = -1,