USB: Add nop functions for !USB case

OTG drivers that use USB core functions like usb_add_hcd, usb_remove_hcd,
etc can be selected for gadget only mode without selecting CONFIG_USB.

Change-Id: Ie4b0c1703622cc9051ad9f50b8901f0005daeeb6
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>