arm64: Fix build with CONFIG_ZONE_DMA=n

Trying to build with CONFIG_ZONE_DMA=n leaves visible references
to the now-undefined ZONE_DMA, resulting in a syntax error.

Hide the references behind an #ifdef instead of using IS_ENABLED.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>