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Instead of calling dmam_alloc_coherent() once for each 4-bit DMA
hardware descriptor, we can have them both in a physical memory page, as
long as they are aligned to 16 bytes. This reduces memory consumption,
and will make it easier to add more DMA descriptors in the future.

Note that the old code would not create the F0 descriptor on SoCs that
don't support multiple planes. We don't care, because:
- we don't use more memory by allocating two descriptors instead of a
  single one;
- the only SoC that does not support multiple planes (JZ4740) still has
  two independent DMA channels, for an unknown reason.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200926170501.1109197-4-paul@crapouillou.net