arm: xen: mm: use __GPF_DMA32 for arm64

arm64 shares some code under arch/arm/xen, including mm.c.
However ZONE_DMA is removed by commit
ad67f5a6("arm64: replace ZONE_DMA with ZONE_DMA32").

So add a check if CONFIG_ZONE_DMA32 is enabled use __GFP_DMA32.
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>

arch/arm/xen/mm.c

@@ -28,7 +28,10 @@ unsigned long xen_get_swiotlb_free_pages(unsigned int order)
 
 	for_each_memblock(memory, reg) {
 		if (reg->base < (phys_addr_t)0xffffffff) {
-			flags |= __GFP_DMA;
+			if (IS_ENABLED(CONFIG_ZONE_DMA32))
+				flags |= __GFP_DMA32;
+			else
+				flags |= __GFP_DMA;
 			break;
 		}
 	}