spi: spi-sun6i: implement DMA-based transfer mode

spi: spi-sun6i: implement DMA-based transfer mode

DMA-based transfer will be enabled if data length is larger than FIFO size
(64 bytes for A64). This greatly reduce number of interrupts for
transferring data.

For smaller data size PIO mode will be used. In PIO mode whole buffer will
be loaded into FIFO.

If driver failed to request DMA channels then it fallback for PIO mode.

Tested on SOPINE (https://www.pine64.org/sopine/)

Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20201022075221.23332-1-akochetkov@lintech.ru
Signed-off-by: Mark Brown <broonie@kernel.org>