spi: sprd: spi: sprd: Add DMA mode support

Add DMA mode support for the Spreadtrum SPI controller, and we will enable
SPI interrupt to help to complete the SPI transfer work in DMA mode.

Signed-off-by: Lanqing Liu <lanqing.liu@unisoc.com>
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>