[POWERPC] bootwrapper: Add ft_find_node_by_prop_value().

ft_find_node_by_prop_value() finds nodes with the specified
property/value pair.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>