dmaengine: axi-dmac: Allocate hardware descriptors

Change where and how the DMA transfers meta-data is stored, to prepare
for the upcoming introduction of scatter-gather support.

Allocate hardware descriptors in the format that the HDL core will be
expecting them when the scatter-gather feature is enabled, and use these
fields to store the data that was previously stored in the axi_dmac_sg
structure.

Note that the 'x_len' and 'y_len' fields now contain the transfer length
minus one, since that's what the hardware will expect in these fields.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20231215131313.23840-3-paul@crapouillou.net


Signed-off-by: Vinod Koul <vkoul@kernel.org>