dmaengine: axi-dmac: Add support for scatter-gather transfers

Implement support for scatter-gather transfers. Build a chain of
hardware descriptors, each one corresponding to a segment of the
transfer, and linked to the next one. The hardware will transfer the
chain and only fire interrupts when the whole chain has been
transferred.

Support for scatter-gather is automatically enabled when the driver
detects that the hardware supports it, by writing then reading the
AXI_DMAC_REG_SG_ADDRESS register. If not available, the driver will fall
back to standard DMA transfers.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20231215131313.23840-4-paul@crapouillou.net


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