core: Add functions to set properties in live-tree

Implement a set of functions to manipulate properties in a live device
tree:

* ofnode_write_prop() to set generic properties of a node
* ofnode_write_string() to set string properties of a node
* ofnode_set_enabled() to either enable or disable a node

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Reviewed-by: Simon Glass <sjg@chromium.org>