arm64: rename variables used to calculate ZONE_DMA32's size

Let the name indicate that they are used to calculate ZONE_DMA32's size
as opposed to ZONE_DMA.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>