ARM: Fix appended device tree when CONFIG_OFTREE is enabled
When CONFIG_OFTREE is enabled the appended device tree is unflattened
and put into data->of_root_node, but there it is never used again.
To actually use the appended device tree put it into data->oftree
instead.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>