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>