RISC-V: Add conditional macro for zone of DMA32

The DMA32 is for 64-bit usage.
Signed-off-by: Zong Li <zong@andestech.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/mm/init.c

@@ -28,7 +28,9 @@ static void __init zone_sizes_init(void)
 {
 	unsigned long max_zone_pfns[MAX_NR_ZONES] = { 0, };
 
+#ifdef CONFIG_ZONE_DMA32
 	max_zone_pfns[ZONE_DMA32] = PFN_DOWN(min(4UL * SZ_1G, max_low_pfn));
+#endif
 	max_zone_pfns[ZONE_NORMAL] = max_low_pfn;
 
 	free_area_init_nodes(max_zone_pfns);