dm: core: Allow writing to a flat tree with ofnode

dm: core: Allow writing to a flat tree with ofnode

In generally it is not permitted to implement an ofnode function only for
flat tree or live tree. Both must be supported. Also the code for
live tree access should be in of_access.c rather than ofnode.c which is
really just for holding the API-conversion code.

Update ofnode_write_prop() accordingly and fix the test so it can work
with flat tree too.

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