usb: add device connection/disconnection detection

Provide a function to detect USB device insertion/removal in order to
avoid having to do USB enumeration in a tight loop when trying to detect
peripheral hotplugging.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>

Reviewed-by: Stefan Reinauer <reinauer@chromium.org>
Tested-by: Stefan Reinauer <reinauer@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/usb_hub.c b/common/usb_hub.c
index af39c76..be01f4f 100644
--- a/common/usb_hub.c
+++ b/common/usb_hub.c
@@ -79,7 +79,7 @@
 			data, sizeof(struct usb_hub_status), USB_CNTL_TIMEOUT);
 }
 
-static int usb_get_port_status(struct usb_device *dev, int port, void *data)
+int usb_get_port_status(struct usb_device *dev, int port, void *data)
 {
 	return usb_control_msg(dev, usb_rcvctrlpipe(dev, 0),
 			USB_REQ_GET_STATUS, USB_DIR_IN | USB_RT_PORT, 0, port,