spi: sprd: Add the SPI irq function for the SPI DMA mode

The SPI irq event will use to complete the SPI work in the SPI DMA mode,
so this patch is a preparation for the following DMA mode support.

Moreover the SPI interrupt can be fired when removing the SPI controller,
so we should make sure the SPI controller has stopped the queue in
remove function before freeing the SPI irq.

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>