[POWERPC] Silence an annoying boot message

vmemmap_populate will printk (with KERN_WARNING) for a lot of pages
if CONFIG_SPARSEMEM_VMEMMAP is enabled (at least it does on iSeries).
Use pr_debug for it instead.

Replace the only other use of DBG in this file with pr_debug as well.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>