libpayload: Implement usb_exit

libpayload: Implement usb_exit

So far it was empty and never published. It now exists and shuts down
all controllers (esp. EHCI which resets the port routers).

Change-Id: I81e355e8a05778d6397675417b085a094a6f48ee
Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/397
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>