[POWERPC] bootwrapper: Add dt_ops methods.

Add get_parent, create_node, and find_node_by_prop_value to dt_ops.
Currently only implemented by flatdevtree_misc.

Also, add a _str convenience wrapper for setprop.

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>