dtoc: Tidy up property-offset handling

If a property does not yet have an offset, then that means it exists in
the cache'd fdt but has not yet been synced back to the flat tree. Use
the dirty flag for this so we don't need to check the offset too. Improve
the comments for Prop and Node to make it clear what an offset of None
means.

Also clear the dirty flag after the property is synced.

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