[S390] Optional ZONE_DMA for s390.

Disable ZONE_DMA on 31-bit. All memory is addressable by all
devices and we do not need any special memory pool.
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/Kconfig

@@ -8,8 +8,8 @@ config MMU
 	default y
 
 config ZONE_DMA
-	bool
-	default y
+	def_bool y
+	depends on 64BIT
 
 config LOCKDEP_SUPPORT
 	bool

arch/s390/mm/init.c

@@ -141,7 +141,9 @@ void __init paging_init(void)
 	__raw_local_irq_ssm(ssm_mask);
 
 	memset(max_zone_pfns, 0, sizeof(max_zone_pfns));
+#ifdef CONFIG_ZONE_DMA
 	max_zone_pfns[ZONE_DMA] = PFN_DOWN(MAX_DMA_ADDRESS);
+#endif
 	max_zone_pfns[ZONE_NORMAL] = max_low_pfn;
 	free_area_init_nodes(max_zone_pfns);
 }