OF: fix of_find_node_by_path() assumption that of_allnodes is root

of_find_node_by_path() is borked because of_allnodes is not guaranteed to
contain the root of the tree after using any of the dynamic update functions
because some other nodes ends up as of_allnodes.

Fixes: c22e650e of: Make of_find_node_by_path() handle /aliases
Reported-by:  <pantelis.antoniou@konsulko.com>
Signed-off-by: Frank Rowand <frank.rowand@sonymobile.com>
Signed-off-by: Rob Herring <robh@kernel.org>