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>