arm64: mm: keep low RAM dma zone

arm64: mm: keep low RAM dma zone

Commit ba0fb44aed47 ("dma-mapping: replace zone_dma_bits by
zone_dma_limit") optimistically assumed that device-tree dma-ranges
property describes the system DMA limits. That assumption ignores DMA
limits of individual devices that are not encoded in device tree.

Commit 833bd284a45 ("arm64: mm: fix DMA zone when dma-ranges is
missing") fixed part of the problem for platforms that do not provide
dma-ranges at all. However platforms like SM8550-HDK provide DMA bus
limit, but have devices with stronger DMA limits.
of_dma_get_max_cpu_address() does not take device limitations into
account.

These platforms implicitly rely on DMA zone in low 32-bit RAM area.
Until we find a better way to figure out the optimal DMA zone range,
restore the low RAM DMA zone we had before commit ba0fb44aed47.

Fixes: ba0fb44aed47 ("dma-mapping: replace zone_dma_bits by zone_dma_limit")
Closes: https://lore.kernel.org/r/1a0c7282-63e0-4add-8e38-3abe3e0a8e2f@linaro.org
Reported-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Suggested-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8550-HDK
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>