dtoc: Tweak ordering of fdt-offsets refreshing

dtoc: Tweak ordering of fdt-offsets refreshing

Once the tree has been synced, thus potentially moving things around in the
fdt, we set _cached_offsets to False so that a refresh will happen next
time a property is accessed.

This 'lazy' refresh doesn't really save much time, since refresh is a very
fast operation, just a single walk of the tree. Also, having the refresh
happen in the bowels of property access it makes it harder to figure out
what is going on.

Simplify the code by always doing a refresh before and after a sync. Set
_cached_offsets to True immediately after this, in the Refresh() function,
since this makes more sense than doing it in the caller.

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