[S390] mm: add ZONE_DMA to 31-bit config again

Add ZONE_DMA to 31-bit config again. The performance gain is minimal
and hardly anybody cares anymore about a 31-bit kernel.
So add ZONE_DMA again to help with SLAB_CACHE_DMA removal for
!CONFIG_ZONE_DMA configurations.

Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index ff2d237..9fab2aa 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -2,7 +2,7 @@
 	def_bool y
 
 config ZONE_DMA
-	def_bool y if 64BIT
+	def_bool y
 
 config LOCKDEP_SUPPORT
 	def_bool y