powerpc/bootwrapper: Use the child-bus #address-cells to decide which range entry to use

The correct #address-cells was still used for the actual translation,
so the impact is only a possibility of choosing the wrong range entry
or failing to find any match.  Most common cases were not affected.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>