usb boot: add xhci mmio example

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20201013091019.22029-1-kraxel@redhat.com
diff --git a/src/boot.c b/src/boot.c
index bd78fb9..1effd80 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -423,7 +423,9 @@
 {
     if (!CONFIG_BOOTORDER)
         return -1;
-    // Find usb - for example: /pci@i0cf8/usb@1,2/storage@1/channel@0/disk@0,0
+    // Find usb - examples:
+    //   pci:  /pci@i0cf8/usb@1,2/storage@1/channel@0/disk@0,0
+    //   mmio: /sysbus-xhci@00000000fe900000/storage@1/channel@0/disk@0,0
     char desc[256], *p;
 
     if (usbdev->hub->cntl->pci)