dm: core: Allow obtaining a node offset in the same tree

In some cases we want to obtain an ofnode in the same tree as a different
ofnode, such as when looking up a subnode. At present this is trivial,
since there is only one tree. When there are multiple trees, this
implementation will change.

Also move the ofnode_to_offset() function up higher in the header file,
since we will need to provide a different implementation with multiple
trees.

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