[POWERPC] wrapper: Treat NULL as root node in devp_offset; add devp_offset_find()

Many operations, as currently used in the wrapper, assume they can
pass NULL and have it be treated as the root node.  However, libfdt-wrapper
converts NULL to -1, which is only appropriate when searching for nodes,
and will cause an error otherwise.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>