USB: Add empty functions in otg.h

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: Maulik Mankad <x0082077@ti.com>
Cc: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Cc: Ajay Kumar Gupta <ajay.gupta@ti.com>
Acked-by: Olof Johansson <olof@lixom.net>
Acked-by: Felipe Balbi <felipe.balbi@nokia.com>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>