[POWERPC] bootwrapper: dt_xlate_range() bugfixes

1. The check whether ranges fits in the buffer was using elements rather
than bytes.
2. Empty ranges were not properly treated as transparent, and missing
ranges were treated as transparent.
3. The loop terminated when translating from the root rather than to.  Once
bug #2 was fixed, it failed due to a missing ranges in the root node.
4. In decoding the ranges property, the #size-cells used was that of
the parent, not the child.

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>