dma: edma: Declare DMA_SLAVE_BUSWIDTH_3_BYTES as supported buswidth

eDMA can be configured for 3bytes word size for source and destination.

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>