mm: refresh ZONE_DMA and ZONE_DMA32 comments in 'enum zone_type'

These zones usage has evolved with time and the comments were outdated.
This joins both ZONE_DMA and ZONE_DMA32 explanation and gives up to date
examples on how they are used on different architectures.

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