[POWERPC] bootwrapper: Use map_string() instead of lookup_string() in ft_prop().

When adding a property, the property name should be added to the string
table if it doesn't already exist.  map_string() does that;
lookup_string() will fail instead.

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>