powerpc: Fix uninitialised error in numa.c

commit 82b2521d257b5c0efd51821cf5fa306e53bbb6ba upstream.

chroma_defconfig currently gives me this with gcc 4.6:
  arch/powerpc/mm/numa.c:638:13: error: 'dm' may be used uninitialized in this function [-Werror=uninitialized]

It's a bogus warning/error since of_get_drconf_memory() only writes it
anyway.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>