[POWERPC] Fix vmemmap warning in init_64.c

Use the right printk format to silence the following warning.

  CC      arch/powerpc/mm/init_64.o
arch/powerpc/mm/init_64.c: In function 'vmemmap_populate':
arch/powerpc/mm/init_64.c:243: warning: format '%p' expects type 'void *', but argument 4 has type 'long unsigned int'

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>