powerpc: Const-qualify Device Node Argument to DCR Resource Extent API

Add const qualifier to device_node argument for
dcr_resource_{start,len} as of_get_property also const-qualifies this
argument.

Signed-off-by: Grant Erickson <gerickson@nuovations.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>