We calculate the full path of the current node using fdt_get_path which
takes a lot of time since libfdt has to iterate over the dtb several times.
Speed this up by creating and using a of_find_child function which does
not have to iterate over the whole tree but only over its direct children.
On an i.MX51 board this speeds up unflatten the tree from 60ms to 3.7ms.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>