dma: Support for 3 bytes word size

Add DMA_SLAVE_BUSWIDTH_3_BYTES to dma_slave_buswidth for engines and users
to select 3 bytes as bus width.
For example eDMA can be configured to use 3bytes mode and in audio we have
formats stored on 3bytes in memory (_XXX_3LE) where this new bus width can
be used.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Acked-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@linaro.org>

include/linux/dmaengine.h

@@ -299,6 +299,7 @@ enum dma_slave_buswidth {
 	DMA_SLAVE_BUSWIDTH_UNDEFINED = 0,
 	DMA_SLAVE_BUSWIDTH_1_BYTE = 1,
 	DMA_SLAVE_BUSWIDTH_2_BYTES = 2,
+	DMA_SLAVE_BUSWIDTH_3_BYTES = 3,
 	DMA_SLAVE_BUSWIDTH_4_BYTES = 4,
 	DMA_SLAVE_BUSWIDTH_8_BYTES = 8,
 };