Add empty functions for usb_nop_xceiv_register()
and usb_nop_xceiv_unregister() in otg.h so that
these functions can be called even when
CONFIG_NOP_USB_XCEIV is not enabled.

It allows to remove ifdef's from board file.
Signed-off-by: NMaulik Mankad <x0082077@ti.com>
Cc: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Cc: Ajay Kumar Gupta <ajay.gupta@ti.com>
Acked-by: NOlof Johansson <olof@lixom.net>
Acked-by: NFelipe Balbi <felipe.balbi@nokia.com>
Acked-by: NGreg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: NTony Lindgren <tony@atomide.com>