[POWERPC] Improved documentation of device tree 'ranges'

This clarifies how 'ranges' properties are used, and in particular,
how to get an identity mapping.

Signed-off-by: Stephen Neuendorffer <stephen.neuendorffer@xilinx.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>