device_romstage: Add a way to move to the next device

When trying to loop through all the devices in romstage, there was
no function to just go from one to the next.

This allows an easy way to go all the way down the chain of devices.

Change-Id: Id205b24610d75de060b0d48fa283a2ab92d1df0a
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: http://review.coreboot.org/5732
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marc.jones@se-eng.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>