powerpc: Fix uninitialised error in numa.c

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>
cc: <stable@kernel.org> [v3.3+]
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/numa.c

@@ -635,7 +635,7 @@ static inline int __init read_usm_ranges(const u32 **usm)
  */
 static void __init parse_drconf_memory(struct device_node *memory)
 {
-	const u32 *dm, *usm;
+	const u32 *uninitialized_var(dm), *usm;
 	unsigned int n, rc, ranges, is_kexec_kdump = 0;
 	unsigned long lmb_size, base, size, sz;
 	int nid;