USB: add new configuration variable CONFIG_PCI_OHCI_DEVNO

In case of several PCI USB controllers on a board this variable
specifys which controller to use.
See doc/README.generic_usb_ohci for details.

Signed-off-by: Sergei Poselenov <sposelenov@emcraft.com>
diff --git a/doc/README.generic_usb_ohci b/doc/README.generic_usb_ohci
index c44c501..147ea51 100644
--- a/doc/README.generic_usb_ohci
+++ b/doc/README.generic_usb_ohci
@@ -51,6 +51,12 @@
 
 	CONFIG_PCI_OHCI
 
+If you have several USB PCI controllers, define
+
+	CONFIG_PCI_OHCI_DEVNO: number of the OHCI device in PCI list
+
+If undefined, the first instance found in PCI space will be used.
+
 PCI Controllers need to do byte swapping on register accesses, so they
 should to define: