event: Add an event for device tree fixups

At present there is a confusing array of functions that handle the
device tree fix-ups needed for booting an OS. We should be able to switch
to using events to clean this up.

As a first step, create a new event type and call it from the standard
place.

Note that this event uses the ofnode interface only, since this can
support live tree which is more efficient when making lots of updates.

Signed-off-by: Simon Glass <sjg@chromium.org>